Java Programming for Beginners Learn the fundamentals of programming with Java Mark Lassoff BIRMINGHAM - MUMBAI Java Views 813 Downloads 207 File size 29MB Report DMCA / Copyright DOWNLOAD FILE Recommend Stories Citation previewJava Programming for Beginners Learn the fundamentals of programming with Java Mark Lassoff BIRMINGHAM - MUMBAI Java Programming for Beginners Copyright © 2017 Packt Publishing All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior written permission of the publisher, except in the case of brief quotations embedded in critical articles or reviews. Every effort has been made in the preparation of this book to ensure the accuracy of the information presented. However, the information contained in this book is sold without warranty, either express or implied. Neither the author, nor Packt Publishing, and its dealers and distributors will be held liable for any damages caused or alleged to be caused directly or indirectly by this book. Packt Publishing has endeavored to provide trademark information about all of the companies and products mentioned in this book by the appropriate use of capitals. However, Packt Publishing cannot guarantee the accuracy of this information. First published: October 2017 Production reference: 1301017 Published by Packt Publishing Ltd. Livery Place 35 Livery Street Birmingham B3 2PB, UK. ISBN 978-1-78829-629-8 www.packtpub.com Credits Author Mark Lassoff Project Editor Suzanne Coutinho Acquisition Editor Dominic Shakeshaft Proofreader Safis Editing Content Development Editor Venugopal Commuri Indexer Pratik Shirodkar Technical Editor Nidhisha Shetty Graphics Tania Dutta Copy Editor Gladson Monteiro Production Coordinator Arvindkumar Gupta About the Author Mark Lassoff, according to his parents' frequent claim, was born to be a programmer. In the mid-eighties, when the neighborhood kids were outside playing kickball and throwing snowballs, Mark was hard at work on his Commodore 64, writing games in the BASIC programming language. Computers and programming continued to be his strong interest in college, where Mark majored in communications and computer science. After completing his college career, Mark worked in the software and web development departments at several large corporations. In 2001, on a whim, while his contemporaries were conquering the dot com world, Mark accepted a position training programmers in a technical training center in Austin, Texas. It was there that Mark fell in love with teaching programming, which has been his passion ever since. Today Mark is a top technical trainer, traveling the country providing training for software and web developers. Mark's training clients include the Department of Defense, Lockheed Martin, Discover Card Services, and Kaiser Permanente. He has consulted for companies such as Dell, Target, and Lockheed Martin, and government agencies including the US House of Representatives. In addition to traditional classroom training and consulting, Mark releases video tutorial training for aspiring programmers on his websites. He lives in coastal Connecticut near the Connecticut River, where he is in the process of redecorating his house. www.PacktPub.com For support files and downloads related to your book, please visit www.PacktPub.com. Did you know that Packt offers eBook versions of every book published, with PDF and ePub files available? You can upgrade to the eBook version at www.PacktPub.com and as a print book customer, you are entitled to a discount on the eBook copy. Get in touch with us at [email protected] for more details. At www.PacktPub.com, you can also read a collection of free technical articles, sign up for a range of free newsletters and receive exclusive discounts and offers on Packt books and eBooks. https://www.packtpub.com/mapt Get the most in-demand software skills with Mapt. Mapt gives you full access to all Packt books and video courses, as well as industry-leading tools to help you plan your personal development and advance your career. Why subscribe? 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Features of Java Java applications Setting up your development environment Installing JDK Installing the NetBeans IDE Writing our first Java program Creating a new project Creating a Java class Writing the code Java comments The main() function Printing a string Java syntax Executing our program How to interpret errors detected by NetBeans? The code completion feature Summary Chapter 2: Understanding Typed Variables Integer variables The solution of variables Memory allocation for integer variables Floating point variables Behavior difference between integer and float data types Type casting The double data type The Math class library Using the pow() function Importing class libraries Char variables Strings The concatenation operator The toUpperCase() function 1 7 7 8 9 10 10 12 14 14 16 18 19 19 20 20 22 24 28 30 31 32 34 37 40 41 45 45 47 48 51 51 55 56 57 The replace() function Escape sequences Summary 60 61 63 Chapter 3: Branching 64 Understanding if statements Conditional statements Using if statements Using else statements Complex conditionals The contains function Complex conditional statements The boolean variable Switch, case, and break Program using switch, case, and break While and do...while loops While loops The do...while loops For loops Summary Chapter 4: Data Structures 65 68 69 71 74 75 77 80 81 83 87 89 93 95 98 99 Working with arrays Declaring and initializing an array Assigning values to an array Array example in NetBeans Creating an array Creating a for loop Printing the alphabet Default initialization of arrays in Java Multidimensional arrays A multidimensional array example in NetBeans Creating a multidimensional array Using nested loops for multidimensional arrays Assigning different colors to our chessboard Printing the chessboard ArrayLists An ArrayList example in NetBeans Creating an ArrayList Getting user input Printing the ArrayList of user input Giving control to the user Analyzing ArrayIndexOutOfBoundsException [ ii ] 100 100 101 102 103 104 105 106 107 108 109 109 111 112 115 115 116 117 117 118 122 Maps Creating a Map Assigning values to our Map Fetching information from our Map Summary Chapter 5: Functions 124 127 128 129 130 131 The basics of Java functions Methods Temperature conversion program Setting up the control flow Exploring separate cases - C, K, and F Celsius type Kelvin type Fahrenheit type Executing the first part of the program Executing the second part of the program Final touch to the program Advanced Java functions Operating java variables Using variables in the program Understanding passing parameters Summary Chapter 6: Modeling with Object-Oriented Java Creating classes and objects Creating custom classes 131 131 132 134 135 136 137 138 139 141 143 144 146 148 152 154 155 156 158 160 163 165 171 174 Creating member variables Creating member functions Creating constructors Types of constructors Summary Chapter 7: More Object-Oriented Java Inheritance Creating a superclass The is-a relationship 175 175 179 180 182 185 186 187 188 191 Overriding Data structure The abstract method The protected method Abstract Summary [ iii ] Chapter 8: Useful Java Classes 192 The Calendar class Subclasses of Calendar Fetching the current day, month, and year How Calendar works String functionality String literals versus String objects String functions The replace() function The format() function Exceptions Analyzing the console exception messages Handling exceptions The try and catch blocks The finally block The Object class Overriding the equals() method Other Object methods Primitive classes Summary 193 194 197 200 204 205 207 208 209 211 213 214 214 217 219 221 223 224 227 Chapter 9: File Input and Output 228 Writing data to files The FileWriter class 228 229 233 236 237 238 242 243 246 250 Relieving resources using the close() method The BufferedWriter class Reading data from files The FileReader and BufferedReader class The Serializable class Serializing an object Deserializing an object Summary Chapter 10: Basic GUI Development Swing GUIs The JFrame class Setting the size of our window Adding a label Closing our application The SwingUtilities class A visual GUI editor tool – palette [ iv ] 251 251 252 254 255 257 258 260 How palette works Event handling Adding a button Adding functionality to our button Summary Chapter 11: XML 261 267 268 268 274 275 Reading XML data Parsing XML data Accessing Jane's XML element Printing Jane's car details Writing XML data Java code to write XML data Modifying XML data Summary Index 275 279 281 285 289 291 293 295 296 [v] Preface Whether this is your first foray into high-level object-oriented programming languages such as Java, or if you've been programming for some time and you're just looking to add Java to your repertoire, or even if you've never touched a line of code in your life, this book is designed to accommodate you. We'll move quickly and we'll not shy away from heavy subjects, but we'll begin this book from the very ground up and learn the concepts behind object-oriented programming as we go. I will consider the book to be successful if it helps you understand what Java programming is about, why it is important, and how you can start developing Java applications in NetBeans. I will be equally happy if Java becomes your favorite programming language! What this book covers Chapter 1, Getting Started with Java, teaches you what Java is and shows its features. We will see the vastness of Java's application by looking at various fields it is used in. We will walk through the steps to install the Java Development Kit. We will then set up a development environment called NetBeans for writing Java programs, and execute these programs. We'll also see how to use NetBeans and write our first Java program in it. You'll also learn to use NetBeans to detect errors and rectify them. Chapter 2, Understanding Typed Variables, shows what variables are and how they are important for creating better programs. We'll look at some of the primitive data types of Java, int, long, float, char, and double, in detail. You'll also learn about the String class and two of its manipulation methods. Chapter 3, Branching, basically portrays how to work with conditional if...else statements, running through complex conditionals using functions such as contains, complex, and boolean. We will go through the intricacies of switch, case, and break with the help of programs; we'll also deep dive into using loop functionality with while, do...while, and for loops. Preface Chapter 4, Data Structures, discusses arrays and presents an example that prints the English alphabet using an array. Then, we'll take a look at multidimensional arrays and write a program that creates a two-dimensional chess board. We'll walk through what ArrayList is and how it enhances the functionality of arrays. We'll write a program using ArrayList with functionality that would have been quite difficult to implement using an array. Finally, you'll learn about Maps and implement an example to understand them better. Chapter 5, Functions, helps you to start with some of the basic Java functions, such as methods, and then move on to understanding you advanced Java functions. As you become a more experienced programmer, you'll begin to internalize concepts like these and you won't have to really think about them explicitly when you're writing day-to-day code. For now though, there are some logical shortcuts that we can use to keep from getting too tripped up. Chapter 6, Modeling with Object-Oriented Java, teaches how to create classes and objects in Java. We will also walk through creating custom classes, member variables, and member functions. We will also look into a very special member assigned to our custom classes, the constructor, and types of constructor. Chapter 7, More Object-Oriented Java, reveals some intricacies of object-oriented programming, precisely, those using the concept of Inheritance, by creating something called as superclass and subclass, and establishing an is-a relationship between them. You'll learn how to use key aspects such as overriding subclass and super class, data structures, and the Protected method. We also get to know how an abstract method works, in detail. Chapter 8, Useful Java Classes, presents the Calendar class, which is used to work with dates and times. You will learn about the String class in detail. We'll also see what exceptions are and how to handle them to make our programs more robust. Then, we will walk through the Object class and some of its methods. Finally, we will take a look at primitive classes in Java. Chapter 9, File Input and Output, walks you through the concept of writing and reading data files, where we'll see the usage of the FileWriter and FileReader classes and learn to free up resources using the close () method. You'll also learn to catch an exception and handle it. Then, we'll see how to use the BufferedWriter and BufferedReader classes to wrap the FileWriter and FileReader classes, respectively. Lastly, we will talk about one more aspect of I/O, the Serializable class. We will analyze what serialization is and its use with respect to serializing and deserializing objects. [2] Preface Chapter 10, Basic GUI Development, shows the basic functions of the GUI in NetBeans. We will learn how to create an application window with the jFrame class by setting its size, adding labels to it, and closing our application. Then we'll talk about the working of palette and components available in it. Lastly, we'll learn about triggering events by adding a button and adding functionality to it. Chapter 11, XML, explains the Java code to read an XML file into a Document object. You'll also learn how to parse XML data using Java. Finally, you'll learn how to write and modify XML data in Java. What you need for this book For this book, you'll need a Java Development Kit (JDK) and NetBeans Who this book is for This book is for anyone wanting to begin learning the Java language, whether you're a student, casual learner, or existing programmer looking to add a new language to your skillset. No previous experience of Java or programming in general is required. Conventions In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning. Code words in text, database table names, folder names, filenames, file extensions, path names, dummy URLs, user input, and Twitter handles are shown as follows: "The Source Packages file is where we'll be writing our code." A block of code is set as follows: public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World!"); } } [3] Preface When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold: public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World!"); } } Any command-line input or output is written as follows: java -jar WritingToFiles.jar New terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "Choose the Download button below the Java SE column." Warnings or important notes appear in a box like this. Tips and tricks appear like this. 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Questions You can contact us at [email protected] if you are having a problem with any aspect of the book, and we will do our best to address it. [6] 1 Getting Started with Java Whether this is your first foray into high-level object-oriented programming languages, such as Java, or if you've been programming for some time and you're just looking to add Java to your repertoire, or even if you've never touched a line of code in your life, this book is designed to accommodate you. We're going to move quickly, and we're not going to shy away from heavy subjects; however, we're going to begin this book from the very ground up and learn about the concepts behind object-oriented programming as we go. In this chapter, we'll understand what Java is and also look at its features. We'll then follow a step-by-step process to set up a development environment to enable us to write and execute Java programs. Once we accomplish this, we'll write our very first Java program and run it. Lastly, we'll look at what to do when we get an error. Specifically, we'll cover the following topics: What is Java Features and applications of Java Installing JDK Installing the NetBeans IDE Writing HelloWorld.java NetBeans' error detection capabilities What is Java? Java was developed by Sun Microsystems in 1995, but it has stood the test of time and remains highly relevant and widely used to this day. So what exactly is Java? Java is a highlevel, general-purpose object-oriented programming language. Getting Started with Java Features of Java The following are Java's main features: High level and general purpose: Rather than being created to accomplish one very specific task, Java allows us to write computer-readable instructions in an open-ended environment. Because it's not really feasible, or even desirable, for every computer system to have its own specialized programming language, the vast majority of the code is written in high-level, general-purpose languages such as Java. Object-oriented: Java is also what we call an object-oriented language. While we won't get into the specifics of objects and classes until a bit later in this book, know for now that objects allow us to define modular entities within our program that make them much more human-readable and much more manageable to create large-scale software projects. A firm grasp of objectoriented concepts is absolutely essential for any modern software developer. Platform-independent: Lastly, Java was designed with the intention that it be a write once, run anywhere language. This means if you and I both have systems with Java installed and even if our systems are not normally identical--for example, I'm on a Windows machine and you're on a Mac--a Java program on my machine that I give to you will still run essentially the same on your machine without the need for it to be recompiled. [8] Getting Started with Java Compiling a programming language such as Java is the act of taking the human-readable code that we've written and converting it into an interpreted machine-friendly code. Unfortunately, it's usually not very friendly for humans to read or write. To do this, we use a program called the compiler that takes in our code as text and converts it into machine code. Traditionally, we would have to recompile a program for every system that it was going to run on because all systems have a different idea of what their machine code should look like. Java circumvents this issue by compiling all Java programs to the same type of interpreted code called bytecode. A compiled Java program in bytecode can be run by any system in which Java is installed. This is because when we install Java on your system, we also install a Java virtual machine with it that's specific to that system. It is this machine's responsibility to convert the bytecode into the final instructions that head to the processor in that system. By making it the system's responsibility to do this final conversion, Java has created a write once, run anywhere language where I can hand you a Java program and you can run it on your machine while being fairly certain that it's going to run in the same manner that it did on mine. This impressive level of cross-platform support on a language as powerful as Java has made it one of the software developing world's go-to tools for quite some time. Java applications In today's modern times, Java is used to develop desktop applications, web servers, and client-side web applications. It's the native language of the Android operating system, which operates on Android phones and tablets. Java has been used to write video games and is sometimes even ported to smaller devices without a traditional operating system. It remains a huge player in today's technical world, and I'm looking forward to learning it with you. [9] Getting Started with Java Setting up your development environment In this section, we're going to write our first Java program, but before we start coding away, we need to set up an environment that is Java-development friendly. Installing JDK To start off this process, let's download a Java Development Kit (JDK) or a Java SDK. This kit contains libraries and executables that allow us to do lots of different things with Java code. Most importantly, with our SDK installed, we'll be able to compile Java code and then run completed Java programs. You may already have Java installed on your machine; however, unless you've done this explicitly, you probably haven't installed a Java SDK. The version of Java an average user has installed on their machine is called the Java Runtime Environment (JRE). This allows the execution of Java programs, and Java programs won't run on environments without the JRE installed. But the JRE doesn't contain any real development tools, which we're going to need. The good news is that a Java JRE and a Java SDK can exist harmoniously. A Java JRE is really just a subset of the SDK, so if we only have the Java Development Kit installed, which we're about to download, we're going to be fine. If you have downloaded the Java Development Kit in the past, when you actually go to install this kit, Java will let you know that it's already installed and you can skip that portion of the section. For everyone else, check out how to download a development kit: 1. To begin with, navigate to www.oracle.com/technetwork/java/javase/downloads/index.html through your browser. [ 10 ] Getting Started with Java 2. We're going to be using the Java SE, or Standard Edition, Development Kit maintained by Oracle. To acquire this kit, simply go to the Downloads tab, and express that we would like the JDK by selecting that option: [ 11 ] Getting Started with Java Scroll down, check out the license agreement, accept the license agreement, and then download the version of the SDK that's appropriate for your operating system. For me, that's jdk-8u144-windows-x64.exe, listed at the end: 3. Once your download is complete, install it as we would any other program. Choose the default options when appropriate and make sure to take note of the directory to which we will install our development kit. Installing the NetBeans IDE With our Java Development Kit installed, we technically have all the tools we need to start writing Java programs. However, we'd have to compile them through a command line, which can look a little different on different operating systems. So to keep everything simple, let's start learning Java by writing our Java code in an Integrated Development Environment (IDE). This is a software program of its own that helps us write, compile, and run Java programs. We're going to use the NetBeans IDE, which is awesome because it is free, open source, and it's going to operate just about the same on Windows, Mac, and Linux environments. [ 12 ] Getting Started with Java To acquire this IDE, head to netbeans.org/downloads/. You'll see the following page: Because we've downloaded the Java Standard Edition Development Kit, Java SE is the version of NetBeans that we're going to download here. Choose the Download button below the Java SE column. NetBeans should start our download automatically, but if it doesn't, click on the link shown in the following image: [ 13 ] Getting Started with Java Once again, we're going to install NetBeans as we would any other program, choosing the default options when appropriate. Most likely, NetBeans will locate the Java Development Kit on our machine. If it doesn't, it will prompt us for the directory in which we installed the Java Development Kit. Writing our first Java program Hopefully, you have gotten NetBeans installed and have booted it up without any hassle. NetBeans will manage the file structure of our programs, but first, we need to tell NetBeans that we're ready to begin a new project. Creating a new project To create a new project, click on File, then New Project, and choose Java Application: [ 14 ] Getting Started with Java We're going to need to give our project a distinctive name; let's call this one HelloWorld. Then, we can choose a location to put the file. Because this is our very first Java program, we should probably start from as close to scratch as possible. So let's uncheck the Create Main Class option so that NetBeans would give us pretty much a blank project. Then, click on Finish: NetBeans will set up a filesystem for us. We can navigate this filesystem just like we were in a standard filesystem explorer: [ 15 ] Getting Started with Java The Source Packages file is where we'll be writing our code. You'll notice under the Libraries file that the JDK is linked, allowing us to access all of its many library resources: Creating a Java class Once we have created a new project, we should see the Projects, Files, and Services tabs like I have in the following image. Let's look at the Files tab. Whereas the Projects tab is a bit of an abstraction, the Files tab shows us what's actually contained within the filesystem where our HelloWorld project lives: [ 16 ] Getting Started with Java Most importantly, you'll see that the src file here has no files in it. That's because there's no source code associated with our project, so right now it won't do anything. To remedy this, right-click on src, choose New, and then Java Class...: [ 17 ] Getting Started with Java We're going to name our Java Class HelloWorld, just like the name of the project because it is our main class where the program should be entered and start from. Everything else is going to work just fine here for now, so click on Finish and NetBeans will create HelloWorld.java for us. A .java file is essentially a text file, but it should only contain Java code and comments: Writing the code When we told NetBeans to make the HelloWorld.java file, it took some liberties and added some code for us already as shown in the following screenshot: [ 18 ] Getting Started with Java Java comments You'll notice that some of the contents of this document are completely human-readable; these are what we call comments. Any text that appears in a Java file between the /* and */ symbols will be completely ignored by the compiler. We can write whatever we would like in here and it will not affect how our program would operate. For now, let's just delete these comments so that we can deal purely with our Java code. The main() function Java code, like the English language, is read top down and left to right. Even if our project contains many files and many classes, we still need to start reading and executing our code at a specific point. We named this file and class HelloWorld, the same name as our project, because we would like it to be special and contain the public static void main(String[] args) method where the execution of our code will begin. That's quite a mouthful of jargon. For now, just type it out and know that this is the area of our code from where our Java program will begin reading and executing. Once again, this will become much clearer as we begin to learn Java; just know this is the starting point of our Java program. The main() function's code is enclosed in curly brackets: public class HelloWorld { public static void main(String[] args) { } } [ 19 ] Getting Started with Java One of the great things about working in an IDE is that it will highlight which brackets correspond to each other. The brackets allow us to place code within other areas of code. For example, our main() method is contained within the HelloWorld class, and the Java code which we're about to write and execute is going to be contained in our main() method. Line 4, which currently contains nothing, is where our program will look to start reading and executing the Java code. Printing a string Our goal with this HelloWorld program is pretty modest. When it runs, we'd like it to print some text to this output box at the bottom of our screen. When we downloaded the Java SDK, we acquired a library of useful functions, one of which will do just this. This is the println(), or print line, function. When our Java code executes over this function, which it will do right away because it's the first function in our main() method's entry point, the Java code will print some words to our output box. Function names are followed by open and close parentheses. Inside these parentheses, we put information that the functions need to complete their task. The println() method, of course, needs to know what we would like it to print. In Java, a line of text is contained by two double quotation marks and we call it a string. Let's have our program print "Hello World!": Java syntax You might have noticed that NetBeans has been yelling at us for a little bit. There's a light bulb and a red dot on the left and some red jittering under our text, a lot like if we had made a spelling error in some text editors. And that's really what we've done. We've made a syntax mistake. There's something clearly wrong with our Java code and NetBeans knows it. [ 20 ] Getting Started with Java There are two things wrong here. The first is that our code doesn't end with a semicolon. Java doesn't do a good job of reading spaces and carriage returns, so we need to put semicolons at the end of every functional line of code for the same reason that a Morse code operator would send the message "stop" at the end of every line. Let's add a semicolon at the end of our println() statement: NetBeans has become a little more satisfied; the jittering has decreased, but there's still something wrong as shown in the preceding screenshot. The issue is that functions in a programming language, just like files on a computer, have a location where they exist. NetBeans isn't sure where to find the println() function that we've attempted to use. So we simply need to tell NetBeans where this function exists. The full path to the println() function starts from the System package, which includes the out class, which has the definition of the println() function. We write that in Java as System.out.println("Hello World!"); as shown in the following code block. Let's get rid of the extra spaces I created at lines 5, 6, and 7, not because they would affect the way our program runs, but because it doesn't make it look quite as nice. Now we've written our HelloWorld program: public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World!"); } } [ 21 ] Getting Started with Java Executing our program So what do we do with this? Well, as we know, our computer can't read this Java code directly. It must convert it into a computer-readable language. So executing this code becomes a two-step process: 1. Compiling our program: First, we're going to ask NetBeans to build our project. This means that all of the code within our project will be compiled and converted into computer-readable code in a, essentially, computer-readable project: When we press the Build Project button, we'll see a bunch of text in our output box at the bottom of the screen--hopefully the nice BUILD SUCCESSFUL message, followed by the time it took to build the project: [ 22 ] Getting Started with Java 2. Running our program: Once we've built our project, we can press the Run Project button to execute our code and our println statement: NetBeans will then give us the following pop-up box: [ 23 ] Getting Started with Java When we execute a program outside of an IDE, we execute it by launching one of its executable files. Because we're in an integrated development environment right now, NetBeans wants to be sure which of our files we would like to be the entry point of our program. We only have one option here because we've only written one Java class. So let's confirm to NetBeans that HelloWorld is our main class and the main() function in the HelloWorld program will, therefore, be where we start executing our Java program. Then, when we hit OK, our output box will tell us the program has begun to run and our program then prints "Hello World!" to the output box as we intended: There we have it! Now we're Java programmers. Of course, there's more than a little bit left to learn. In fact, HelloWorld in Java is probably the simplest program you'll ever write. Java is extremely powerful, and the reality is we simply can't hope to appreciate all of its intricacies while writing our first program. The really good news is that from this point on, we need to take far fewer leaps of faith and we can begin to build a very solid understanding of Java by taking a step-by-step approach. How to interpret errors detected by NetBeans? As we write more and more complicated Java programs, we're inevitably going to make some mistakes. Some of these mistakes will be significant logic errors or misunderstandings on our part that we might have to further educate ourselves before we can solve them. But, especially while we're starting our programming, we're going to make a lot of small silly errors that are really easy to fix as long as we know where to look. [ 24 ] Getting Started with Java Fortunately, Java compilers are designed to point errors out to us when they come across them. To see this in action, let's simply make our HelloWorld program incorrect by removing the semicolon from the end of the println statement: Now NetBeans red-jitters the line to let us know that it's pretty sure something's wrong, but we can ask our compiler to take a shot at it anyway. If we attempt to build this project, we don't get the COMPILATION SUCCESSFUL message we otherwise would; instead, we get an error message: [ 25 ] Getting Started with Java This error is ';' expected, which is a pretty handy and self-explanatory error message. Of equal importance is the number after the colon in this message, which is 4. This lets us know on what line the compiler came across this error. In NetBeans, if we click on an error message, the IDE will highlight that line of code: [ 26 ] Getting Started with Java If we add in our semicolon, then our program builds successfully as shown in the following screenshot: That's all there is to it. Of course, not all error messages are quite that self-explanatory. For the sake of argument, let's create a slightly more complicated error. What would have happened if we had forgotten to insert one of our parentheses in this program? This is illustrated in the following code: [ 27 ] Getting Started with Java When we press Build Project, we get not one but two errors, even though we really only made one mistake: Our first error is not a statement, then it lets us know the line that it doesn't understand. If we look at the first error for a little bit, we'll probably notice that we're missing a pair of parentheses, so we will be able to fix this error; however, what about the second error? We got ';' expected again even though in this case we really do have a semicolon. Well, once one error has occurred in the program, the compiler's ability to understand the lines of the code gets shattered very quickly. When we're debugging our code, the general rule of thumb is to address only the top error on our list; that's the first error that the compiler came across in our code. We might be able to glean some helpful information from errors further down, but more often than not, they're simply going to be errors generated by the first syntax mistake we made. Nothing too mind-blowing here, but I wanted to point this out to you because being able to track compiler errors can save us a lot of headaches while we're learning to program. The code completion feature While we're talking about NetBeans, let's quickly go over one other IDE feature. Let's say I wanted to write a new line of code and I'm going to use something from the System library: [ 28 ] Getting Started with Java Once I've typed System., NetBeans can suggest valid responses for me. Only one of these, of course, is going to be what I'm looking for. The NetBeans compiler has a lot of helpful features such as these. If you're the kind of person who thinks code completion is awesome, go ahead and leave these tools on. We can do this by going to Tools | Options | Code Completion and checking the features that we'd like: If you'd rather NetBeans behave a little more like a text editor, go ahead and uncheck all the features. There we go, lots of housecleaning in this section, but hopefully quick and not too painful. [ 29 ] Getting Started with Java Summary In this chapter, you learned what Java is and saw its features. We saw the expanse of Java's application by looking at the various fields it is used in. We walked through the steps to install a Java Development Kit. We then set up a development environment called NetBeans for writing Java programs and executing them. We saw how to use NetBeans and wrote our first Java program in it. Next, we saw how to use NetBeans' ability to detect errors for rectifying them. In the next chapter, we'll look at the various Java data types and how to work with variables. [ 30 ] 2 Understanding Typed Variables To create even modest Java programs, we're going to need a way to store and manipulate information. Our chief resource, when doing this, is the variable, and that's what we're going to take a look at in this chapter. We'll look at the different data types in Java and how to use them in our programs. We'll also see the Math class library and one of its functions. Specifically, we'll walk through the following topics: Introduction to variables and why they're needed Integer variables Floating-point variables The Math class library and its pow() function Character variables The String class and its methods Understanding Typed Variables Integer variables To begin, let's create a new project in NetBeans. I'm going to call mine Variables, and this time we'll allow NetBeans to create the main class for us so that we can get to coding as quickly as possible. We need to delete all the comments that are created automatically by NetBeans when we create our new project, just to keep everything as readable as possible, then we'll be good to go: [ 32 ] Understanding Typed Variables The first computers were little more than calculators, and Java, of course, retains this functionality. For example, Java can evaluate 1+1, which will evaluate to 2, of course. However, Java is pretty complicated and designed to do a lot of different things, so we need to provide context to our commands. Here, we tell Java that we'd like it to print the result of 1+1: package variables; public class Variables { public static void main(String[] args) { System.out.println(1+1); } } Our preceding program will run as expected: In addition to some others, Java can perform all the basic arithmetic operations. It can do addition, subtraction, multiplication (for which we use *, not X on our keyboard), and division. If we run the following program with the input of 2 and 3, we'll see four println() commands, all of which will give the proper result of the calculations. We can, of course, change these numbers to be any combination of numbers we see fit: package variables; public class Variables { public static void main(String[] args) { System.out.println(2+3); System.out.println(2-3); System.out.println(2*3); System.out.println(2/3); } } [ 33 ] Understanding Typed Variables The following is the output of the preceding code: Changing these lines manually is kind of a pain and quickly becomes infeasible if we're writing very complicated programs or dynamic programs that take user input. The solution of variables Fortunately, programming gives us a way of storing and retrieving data; this is called the variable. To declare a variable in Java, we first have to specify what kind of variable we're going to be using. Variables come in a number of different types. In this instance, we're content with using whole numbers, that is, numbers that do not have a specified decimal place and aren't fractions. Also, in this case, it's appropriate to use one of Java's primitive types. These are essentially as base level as we can get with information in the Java programming language; just about everything else we work with in Java is built of the primitive types. To declare a variable of the integer primitive type, that is, whole numbers, we use the int keyword, all lowercase. Once we do this, we need to give our variable a name. This is a unique identifier that we'll use to access this piece of information in future. Each variable in our local program should have its own name. Let's call our first variable x and our second variable y: package variables; public class Variables { public static void main(String[] args) { int x; int y; System.out.println(2+3); System.out.println(2-3); System.out.println(2*3); System.out.println(2/3); } } [ 34 ] Understanding Typed Variables We've just written two perfectly legitimate lines of Java code. If we run our program now, we'll see the same output we did before: However, behind the scenes, Java will also be setting aside memory space for our x and y variables. This allocation doesn't affect our println commands because the variables are not referenced in them yet. So let's store some information in our variables. We can reference a variable once we've created it simply by the variable's name. It's important that we do not reference our variable by typing int x again because this is the command for Java to create a brand new variable x, not access the existing variable x: Once we've referenced our variable, we can change its value using the equal sign. So let's set x to 4 and y to 3. Our println commands currently operate with two explicitly declared integers: the numbers 2 and 3. Since x and y are also integers, it stands to reason that we can simply replace the existing numbers with the variables x and y: package variables; public class Variables { public static void main(String[] args) { int x; int y; x = 4; [ 35 ] Understanding Typed Variables y = 3; System.out.println(x+y); System.out.println(x-y); System.out.println(x*y); System.out.println(x/y); } } The following is the output of the preceding code: When our Java code comes to the variables x and y, it will look to see what integer value they have currently been given. It will find the numbers 4 and 3. So if we run our program, we should expect the first println statement, x+y, to evaluate to 4+3, which then evaluates to 7. This is exactly what occurs. So here's something interesting. The last line of our program, in which we divide x by y, isn't evaluating as we might mathematically expect it to. In this line of code, x has the value 4, and y has the value 3, Now 4 divided by 3 equals 1.3, but our program is simply outputting 1. That's because 1.3 is not a valid integer value. Integers are only whole numbers and never fractions or decimal numbers. So, to keep us working with integers, Java simply rounds down any calculations that have fractional portions to their nearest whole number. If we want to work in an environment where we could have fractional results, we would need to use a primitive type other than an integer. Anyway, now that we've set up our println commands to take integer variable input instead of explicit numbers, we can modify the behavior of all four lines of the calculation by simply changing the values of these integer variables. For example, if we wanted our program to run on the input values -10 and 5 (integers can be negative; they just can't have fractional components), all we would need to do is change the values we give to our variables x and y: package variables; public class Variables { [ 36 ] Understanding Typed Variables public static void main(String[] args) { int x; int y; x = -10; y = 5; System.out.println(x+y); System.out.println(x-y); System.out.println(x*y); System.out.println(x/y); } } If we run the preceding code quickly, we will see the expected results: Awesome! You've just learned the very basics of working with both integers and variables in Java. Memory allocation for integer variables Let's go over an edge case and learn a little bit more about how Java thinks. You might remember from earlier, I spoke about how Java sets aside memory when we declare new variables. This is one of the huge advantages of working in a high-level programming language, such as Java. Java abstracts away or automatically takes care of most of the memory management for us. Quite often, this makes writing programs simpler, and we can write shorter, cleaner, and more easily readable code. Of course, it is important that we appreciate what's happening behind the scenes, lest we run into issues. For example, whenever Java sets aside memory for an integer variable, it also sets aside the same amount of memory for all integer variables. This means there's a maximum and minimum value that Java could ever conceivably store in an integer variable. The maximum integer value is 2147483647 and the minimum integer value is 2147483648. [ 37 ] Understanding Typed Variables So let's do an experiment. What happens if we attempt to store and print out an integer variable that is one larger than the maximum value? To start with, let's simplify our program. We're simply going to assign a value, one higher than possible, to the variable x: When we attempt to do this, NetBeans yells at us. It's got some logic built in that attempts to stop us from making this very basic and common mistake. If we were to attempt to compile this program, we would also get an error. However, we want to make this mistake in the name of science, so we're going to trick NetBeans. We're going to set the value of our variable x to the largest possible integer value, and then in the next line of our code, we're going to set the value of x to be one higher than what x is currently, that is, x=x+1. Actually, there's a nifty little shorthand we can use for this: x=x+1 is equivalent to x++. OK, so when we run this program, which will sneak by the compiler and NetBeans, and do our addition at runtime, we attempt to print out an integer value that is one plus the highest integer value that Java can store in a memory location: package variables; public class Variables { public static void main(String[] args) { int x; x = 2147483647; x++; System.out.println(x); } } [ 38 ] Understanding Typed Variables When we run the preceding program, we get the following negative number: This number happens to be the smallest number that we could ever store in an integer. This makes some sort of visual sense. We've gone so far positive, or to the right, on our integer number line, that we've arrived at the leftmost or the most negative point. Of course, in a mathematical sense, this could get pretty confusing pretty quickly. It's unlikely that we're ever going to write programs that will need integer numbers higher than this value. However, if we do, we certainly need to be aware of this issue and circumvent it, using a variable type that can handle larger values. The long variable type is just like an integer but we need to allocate more memory for it: package variables; public class Variables { public static void main(String[] args) { long x; x = 2147483647; x++; System.out.println(x); } } When we run the preceding program, we will get a mathematically accurate result: [ 39 ] Understanding Typed Variables Floating point variables When we're simply counting and manipulating whole objects, integers are fantastic. However, sometimes we need to deal with numbers in a more mathematical sense, and we need a data type that will allow us to express ideas that are not entirely whole numbers. Floating-point numbers, or floats, are a Java primitive type that allow us to express numbers that have decimal points and fractions. In this section, we'll modify some float and integer variables side by side to see how they are similar and different. Let's create a new Java project (you know the drill by now) and call it FloatingPointNumbers. Let's start by declaring two variables: one integer (iNumber) and one float (fNumber). As we know, once we've declared these variables, we're free to modify and assign values to them in our Java program later. This time, let me show you that we can also modify and assign to these variables in the same line that they're declared. So where I have declared my iNumber integer variable, I'm free to immediately give it the value of 5: package floatingpointnumbers; public class FloatingPointNumbers { public static void main(String[] args) { int iNumber = 5; float fNumber; } } Notice that if we try and do something very similar with our float variable, NetBeans will yell at us, by displaying a light bulb and red dot on the left-hand side: In fact, if we attempt to compile our program, we'll get a legitimate compiler error message: [ 40 ] Understanding Typed Variables Let's analyze why this happens. When we use an explicit number in Java, that is, typing out the digits rather than working with a variable, that explicit number is still given a type by Java. So when we type out a number without any decimal places, the type that the number is assumed to be is an integer. So our assignment works just great. However, a number with decimal places is assumed to be of this type and is called double; it's a sister type of the float data type, but it's not quite the same. We'll talk about double a little later. Right now, what we need to do is tell Java to treat 5.5 as a float type number instead of double. To do this, all we need to do is put f after the digits, as follows: float fNumber = 5.5f; You'll see that the bulb and red dot have disappeared. To make sure we get the syntax right, let's give our program some super basic functionality. Let's use System.out.println() to print our integer number and then our floating-point number variable in sequence: System.out.println(iNumber); System.out.println(fNumber); When we build this program, our compiler error goes away, and when we run it, we see the two assigned values as expected. Nothing too exciting there: Behavior difference between integer and float data types Now, rather than assigning explicit values to our variables, let's do some basic arithmetic so we can see how integer and floats, when modified in Java, behave differently. In Java, both float and int are primitive types, the logical building blocks of the programming language. This means we can compare and modify them using mathematical operators, such as division. [ 41 ] Understanding Typed Variables We know that if we attempt to divide one integer by another, we'll always get a whole number as a result, even if the rules of standard mathematics don't make that the expected result. If we divide a floating-point number by another floating-point number, however, we'll get a more mathematically accurate result: package floatingpointnumbers; public class FloatingPointNumbers { public static void main(String[] args) { int iNumber = 5/4; float fNumber = 5.0f/4.0f; System.out.println(iNumber); System.out.println(fNumber); } } The following is the output of the preceding code: Sometimes, Java will let us do things that might not be such a good idea. For example, Java lets us set the value of our floating-point variable fNumber to one integer divided by another instead of one floating-point number divided by another: int iNumber = 5/4; float fNumber = 5/4; [ 42 ] Understanding Typed Variables Because the computation on the right-hand side of our equals sign occurs before the value of our floating-point variable fNumber changes, we're going to see the same output in both the calculations of 5/4. This is because both the 5's and 4's are integer variables. So when we run our program, even though fNumber remains a floating-point number (as we can tell because it prints out with the decimal place), its value is still set to the rounded down whole number of 5/4: Solving this problem is pretty straightforward; we simply need to change one of our integer values to be a floating-point number by appending f to it: int iNumber = 5/4; float fNumber = 5/4.0f; Now the computation will know how to proceed with a decimal place division: This becomes a little trickier and more important to navigate properly when we stop working with explicitly declared numbers and begin working with variables. [ 43 ] Understanding Typed Variables Let's declare two integer variables now. I'll just call them iNumber1 and iNumber2. Now, rather than attempting to set the value of fNumber to one explicitly declared number divided by another, we'll set its value to iNumber1/iNumber2, and we'll just print out the results stored in fNumber: package floatingpointnumbers; public class FloatingPointNumbers { public static void main(String[] args) { int iNumber1 = 5; int iNumber2 = 6; float fNumber = iNumber1/iNumber2; System.out.println(fNumber); } } When we run this program, because once again we're dividing one integer by another, we're going to see the rounding down phenomenon. The value being stored in our floatingpoint variable is 0.0, the rounded down result of 5/6: If we were working with explicitly declared numbers, we would solve this problem by changing one of the two integer numbers to be treated as a floating-point number by simply putting a decimal place and f after it. In this context, using iNumber2f is not an option because rather than thinking that we're asking it to treat iNumber2 as a floating-point number, Java now believes it's looking for a variable called iNumber2f, which certainly doesn't exist within this context. [ 44 ] Understanding Typed Variables Type casting We can achieve a similar result though, using what's called a cast. This is a command in which we ask Java to treat a variable of one type like it is another one. Here we're circumventing Java's natural inclination to treat iNumber1 and iNumber2 as integers. We're stepping in and saying, "You know what Java, treat this number here as a float," and we're assuming some responsibility when we do this. Java will attempt to do what we ask, but if we choose poorly and attempt to cast one object to an object that it cannot, our program will crash. Fortunately, we're working with primitives here, and primitive types know how to act like another type. So, we can cast the variable iNumber1 to temporarily operate like a floatingpoint number by prefacing it with (float): float fNumber = (float)iNumber1/iNumber2; Now if we run our program, we'll see the expected result of 5/6: That's a pretty solid introduction to working with floating-point numbers, which we're going to use just about any time we want to work with numbers in their mathematical sense rather than as integers for counting whole objects. The double data type Let's quickly talk about the double data type. It is a sister type of float. It provides a greater resolution: double numbers can have even more decimal places. But they take up a little more memory. At this point in time, using double or float is almost always a style or personal preference decision. Unless you're working with complicated software that must run at peak memory efficiency, the extra memory space taken up by a double is not very significant. [ 45 ] Understanding Typed Variables To illustrate how double works, let's change the two integer numbers in our FloatingPointNumbers.java program to a double data type. When we only change the names of the variables, the logic of our program doesn't change at all. But when we change the declaration of these variables from declaring integers to doubles, the logic does change. Anyway, when we explicitly declare a number with decimal places, it defaults to being double: Now we need to fix the error. The error occurs because dividing a double data type by another one is going to return a double result. We can solve this issue in two ways: 1. First, we could cast dNumber1 and dNumber2 to floating-point numbers and then divide them: float fNumber = (float) dNumber1/ (float) dNumber2; 2. However, dividing our two double numbers by each other is a perfectly legitimate operation. So why not allow this to occur naturally and then cast the resulting double to a floating-point number, thereby preserving a greater amount of resolution. Just like in algebra, we can break up conceptual blocks of our program that we would like to occur before another block using parentheses: float fNumber = (float) (dNumber1/dNumber2); Now if we run this program, we get the expected result: [ 46 ] Understanding Typed Variables The Math class library A good portion of our time on any software development project is going to be spent teaching our program to solve the types of problems it would comes across on a regular basis. As programmers, we too will run into certain problems time and time again. Sometimes, we'll need to code our own solutions to these problems and hopefully save them for later use. However, more often than not, someone has run into these problems before, and if they've made their solution publicly available, one of our options is to leverage their solution for our own gain. In this section, we'll use the Math class library, which is bundled with the JDK to solve some math problems for us. To start this section, create a brand new NetBeans project (I'm going to name it TheMathLib) and enter the main() function. We're going to write a very simple program. Let's declare a floating-point number variable and give it a value (don't forget the f alphabet at the end of our explicit number to let Java know that we've declared a floatingpoint number), then use System.out.println() to print this value to the screen: package themathlib; public class TheMathLib { public static void main(String[] args) { float number = 4.321f; System.out.println(number); } } OK, there we go: Now, with this program, we'd like to make it really easy to raise our floating-point number to various powers. So, if we simply want to square this number, I guess we could just print out the value of number*number. If we want to cube it, we could print out number*number*number. And, if we want to raise it to the power of six, we could multiply it six times by itself. Of course, this gets unwieldy pretty quickly, and there's certainly a better way. [ 47 ] Understanding Typed Variables Let's leverage the Java Math class library to help us raise numbers to varying exponential powers. Now, I've just told you that the functionality we're looking for lives in the Math class library. This is the kind of push in the right direction you should expect to get from a Google search, or if you're an experienced software developer, you can implement a specific API. Unfortunately, that's not quite enough information for us to start using the functionality of this class library. We don't know the specifics of how it works or even exactly what functionality it offers us. To find this out, we're going to have to look at its documentation. Here's the documentation web page managed by Oracle for the libraries found in our Java Development Kit: docs.oracle.com/javase/7/docs/api/. Among the libraries that show up on the page is java.lang. When we select it, we'll find the Math class that we've been looking for under Class Summary. Once we navigate to the Math class library page, we get two things. First we get some human-friendly text write-up about the library, its history, what its intended uses are, very meta-level stuff. If we scroll down, we see the functionality and methods implemented by the library. This is the nitty-gritty of where we'd like to be: Using the pow() function One of these functions should stick out for us, which is pow(), or the power function. It returns the value of the first argument (double a) raised to the power of the second argument (double b). In short, it will allow us to raise numbers to an arbitrary power: [ 48 ] Understanding Typed Variables Let's get back to coding. Alright, let's employ this pow() function to modify the value of our variable number after we've declared it. We're going to do something along the lines of number = pow, but we need a little more information: How exactly do we employ this pow() function? Well, if we click on our documentation, we'll see that when the pow() function is declared, in addition to its name, there's also, between parentheses, two arguments specified. These arguments, double a and double b, are the two pieces of information the function is requesting before it can operate as expected. Our job, in order to use this function, is to replace the requests double a and double b with actual variables or explicit values so that the pow() function can do its thing. Our documentation tells us that double a should be replaced with the variable or value that we'd like to raise to the power of double b. So let's replace the first type argument with our variable number, which is what we want to raise to an arbitrary power. On that note, number is float not double, and that's going to give us some trouble unless we simply change it to double. So let's do that. For the second argument, we don't have a precreated variable to replace double b with, so let's just use an explicit value, such as 4.0: Notice that I get rid of the double specifier when I call the pow() function. This specifier only exists to let us know what type Java is expecting. [ 49 ] Understanding Typed Variables In theory, the pow() function now has all the information it needs to go ahead and run and raise the value of our number variable to the power of 4. However, NetBeans is still giving us our red warning sign. Right now, that's because NetBeans, and Java by extension, doesn't know where to find this pow keyword. For the same reasons that we need to specify a full path to System.out.println(), we need to specify a full path in which it can find the pow() function for Java. This is the same path we followed to get to the pow() function in our documentation. So let's specify java.lang.Math.pow() as it's path in our code: package themathlib; public class TheMathLib { public static void main(String[] args) { double number = 4.321; number = java.lang.Math.pow(number, 4.0); System.out.println(number); } } Now we're pretty much good to go. Let's utilize the number variable once in our println statement, then we should be able to run our program: We can plug it into our calculator if we want, but I'm pretty confident that our program has outputted the value of 4.321 raised to the power of 4. So this is great! We've just employed external code to not only make our program easier to write, but also to keep it very human-readable. It required much fewer lines of code than it would have otherwise. [ 50 ] Understanding Typed Variables Importing class libraries One thing that's not super human-readable about our program is the long paths to functions such as pow() and println(). Is there a way we could shorten them? There certainly is. If the makers of Java had wanted to, they could have allowed us to call this function by simply typing Math.pow() in all instances. This unfortunately would have some unintended side effects. For example, if there were two libraries linked up to Java and they both declared a Math.pow() function, Java would not know which one to use. Hence, by default, we're expected to link to libraries directly and explicitly. So, if we'd like to just be able to type out something like Math.pow(), we can import a library into the local space that we're working in. We just need to do an import command above our class and the main() function declaration. All the import command takes as input is the path that we'd like Java to look for when it comes across a keyword, such as pow(), that it doesn't immediately recognize. In order to allow us to employ the easier syntax Math.pow() in our program, we simply need to type import java.lang.Math: package themathlib; import java.lang.Math; public class TheMathLib { public static void main(String[] args) { double number = 4.321; number = java.lang.Math.pow(number, 4.0); System.out.println(number); } } There is some special syntax for imports. Let's say we wanted to import all the class libraries in java.lang. To do this, we could replace .Math with .* and make it java.lang.* which translates to "import every library from the java.lang package." I should probably inform you that for those of us working in NetBeans, this import is done by default. However, in this case, we're going to do it explicitly because you may have to do this while working in other Java environments as well. Char variables Programs that manipulate numbers are all well and good, but quite often, we want to be able to work with text and words as well. To help us do this, Java defines the character, or char, the primitive type. Characters are the smallest entity of text that you can work with on a computer. We can think of them to start off with as single letters. [ 51 ] Understanding Typed Variables Let's create a new project; we'll call it Characters.java. We'll start our program by simply defining a single character. We'll call it character1 and we'll assign to it the value of uppercase H: package characters; public class Characters { public static void main(String[] args) { char character1 = 'H'; } } Just as we have to use some extra syntax when defining a floating-point number explicitly, we need some extra syntax when defining a character. To tell Java that we're explicitly declaring a character value here, we surround the letter we would like to assign to our variable with two single quotation marks. The single quotation marks, as opposed to double quotation marks, let Java know that we're working with a character or a single letter, as opposed to trying to use an entire string. Characters can only have single entity values. If we attempt to assign the value of Hi to character1, NetBeans and Java would both let us know that that's not a valid option: Now, let's move on and write a somewhat convoluted program that will nonetheless work pretty well for our example purposes. Let's define five characters. We'll call them character1 through character5. We'll assign each one of them with one of the five letters of the word "Hello," in that order. When these characters are printed together, our output will show Hello. In the second portion of our program, let's use System.out.print() to display these letters on the screen. The System.out.print() code works just the same as System.out.println(), except that it doesn't add a carriage return at the end of our line. Let's have the last command as println so that our output is separated from all of the additional text presented in our console: package characters; public class Characters { public static void main(String[] args) { char character1 = 'H'; char character2 = 'e'; char character3 = 'l'; char character4 = 'l'; char character5 = 'o'; [ 52 ] Understanding Typed Variables System.out.print(character1); System.out.print(character2); System.out.print(character3); System.out.print(character4); System.out.println(character5); } } If we run this program, it greets us. It says Hello and then there is some additional text: That's pretty straightforward. Now let me show you something that will give us a little insight into how our computer thinks about characters. It turns out that we can set the value of character1 not only by explicitly declaring the capital letter H between two single quotation marks, but also by giving it an integer value. Each possible character value has a corresponding number that we can use in lieu of it. If we replace H with the value of 72, we're still going to print out Hello. If we were to use the value 73, one greater than 72, instead of the capital H, we'll now get the capital letter I, as I is the letter that follows H. We have to make sure we don't put 72 between single quotation marks. The best case scenario is that Java recognizes that 72 is not a valid character and that it looks more like two characters, then our program won't compile. If we use a single digit number surrounded by single quotation marks, our program would compile just fine, but we would get the completely unexpected output of 7ello. [ 53 ] Understanding Typed Variables So how do we figure out the numeric value of characters? Well, there's a universal lookup table, the ASCII table, which maps characters to their numeric values: In this section, we've been dealing with columns 1 (Dec) and columns 5 (Chr), which have the decimal number and the character that they are mapped to. You'll notice that while many of these characters are letters, some are keyboard symbols, numbers, and other things, such as tabs. As far as programming languages are concerned, new lines, tabs, and back spaces are all character elements. To see this in action, let's try replacing some of the characters in our program with the decimal value 9, which should correspond to a tab character. If we replace the middle three letters of our word with tabs, as output, we should expect H, three tabs, and o: package characters; public class Characters { public static void main(String[] args) { char character1 = 'H'; [ 54 ] Understanding Typed Variables char character2 = 9; char character3 = 9; char character4 = 9; char character5 = 'o'; System.out.print(character1); System.out.print(character2); System.out.print(character3); System.out.print(character4); System.out.println(character5); } The following is the output of the preceding code: Strings Let's talk about strings in Java. To begin, create a new NetBeans project, name it StringsInJava, and enter the main() function. Then, declare two variables: a character called c and String called s. Right away, it becomes clear to us that String is a little different. You'll notice that NetBeans did not choose to color code our String keyword with blue, as it would have done if we were declaring a variable of a primitive type: [ 55 ] Understanding Typed Variables This is because String, unlike char, is not a primitive type. String is what we call a class. Classes are the backbone of object-oriented programming. Just as we can declare variables of a primitive type, we can also declare variables of a class, which are called instances. In our program, the variable s is an instance of the String class. Unlike variables of primitive types, instances of a class can contain their own special methodologies and functions declared by the class of which they are an instance. In this section, we'll use some of these string-specific methods and functions to manipulate text. But first, let's take a look at what makes the String class so special. As we know, we can pretty much use our character variables and our character literals interchangeably, as we can with just about any other primitive type. The String class also maps interchangeably with the string literal, which is like a character literal but uses double quotation marks and can contain many or no characters at all. Most Java classes do not map to any sort of literal, and our ability to manipulate string literals through the String class is what makes it so valuable. The concatenation operator Another thing strings can do, which most Java classes cannot, is make use of the addition sign (+) operator. If we declare three strings (say, s1, s2, and s3), we can set the value of our third string to be one string plus another string. We can even add a string literal into the mix. Then, we print s3: package stringsinjava; public class StringsInJava { public static void main(String[] args) { char c = 'c'; String s1 = "stringone"; String s2 = "stringtwo"; String s3 = s1+s2+"LIT"; System.out.println(s3); } } [ 56 ] Understanding Typed Variables When we run this program, we'll see these three strings added together in much the manner that we would expect: The toUpperCase() function So I promised you that strings had the functionality not seen in a simple primitive type. To employ this, let's navigate to the String class in our Java documentation at docs.oracle.com/javase/7/docs/api/. Select java.lang shown under Packages, then scroll down and select String from ClassSummary. As with the documentation for all Java classes, the String documentation contains Method Summary, which will tell us about all the functions we can call on an existing String object. If we scroll down in Method Summary, we'll come to the toUpperCase() function that converts all the characters in a string into uppercase letters: Let's employ this function now. Back in NetBeans, we now need to determine the best place in our program to employ our toUpperCase() function: package stringsinjava; public class StringsInJava { public static void main(String[] args) { char c = 'c'; String s1 = "stringone"; String s2 = "stringtwo"; String s3 = s1 + s2 + "LIT"; System.out.println(s3); } } [ 57 ] Understanding Typed Variables We know we need to employ the toUpperCase() function on the s3 string after finalizing the value of s3 in our StringsInJava.java program. We could do either of these two things: Employ the function on the line immediately after finalizing the value of s3 (by simply typing s3.toUpperCase();). Call the function as part of the line where we would print out the value of s3. Instead of printing out the value of s3, we could simply print out the value of s3.toUpperCase(), as shown in the following code block: package stringsinjava; public class StringsInJava { public static void main(String[] args) { char c = 'c'; String s1 = "stringone"; String s2 = "stringtwo"; String s3 = s1+s2+"LIT"; System.out.println(s3.toUpperCase()); } } If you remember from our documentation, the toUpperCase() function requires no arguments. It knows that it's being called by s3, and that's all the knowledge it needs, but we still provide the double empty parentheses so that Java knows we are in fact making a function call. If we run this program now, we'll get the uppercased version of the string, as expected: [ 58 ] Understanding Typed Variables But, it's important we understand what's going on behind the scenes here. The System.out.println(s3.toUpperCase()); code line is not modifying the value of s3 and then printing out that value. Rather, our println statement evaluates s3.toUpperCase() and then prints out the string returned by that function. To see that the actual value of s3 is not modified by this function call, we can print the value of s3 again: System.out.println(s3.toUpperCase()); System.out.println(s3); We can see that s3 keeps its lowercase components: If we want to permanently modify the value of s3, we could do that on the previous line, and we could set the value of s3 to the function's result: package stringsinjava; public class StringsInJava { public static void main(String[] args) { char c = 'c'; String s1 = "stringone"; String s2 = "stringtwo"; String s3 = s1 + s2 + "LIT"; s3 = s3.toUpperCase(); System.out.println(s3); System.out.println(s3); } } The following is the output of the preceding code: [ 59 ] Understanding Typed Variables The replace() function To confirm that we're all on the same page, let's employ one more method from the String class. If we head back to our documentation and scroll up, we can find the String's replace() method: Unlike our toUpperCase() method, which took no arguments, replace() takes two characters as arguments. The function will return a new string where all the instances of the first character (oldChar) we give as an argument are replaced with the second character (newChar) we gave as an argument. Let's utilize this function on our first println() line in StringsInJava.java. We'll type s3.replace() and give our function two characters as arguments. Let's replace the character g with the character o: package stringsinjava; public class StringsInJava { public static void main(String[] args) { char c = 'c'; String s1 = "stringone"; String s2 = "stringtwo"; String s3 = s1 + s2 + "LIT"; s3 = s3.toUpperCase(); System.out.println(s3.replace('g', 'o')); System.out.println(s3); } } If we run our program, of course, nothing happens. This is because by the time we reach the print statement, there are no lowercase g characters, and there are no lowercase g characters left in s3; there are only uppercase G characters. So let's try and replace the uppercase G characters: System.out.println(s3.replace('G', 'o')); System.out.println(s3); [ 60 ] Understanding Typed Variables Now if we run our program, we see the replacement happen on the first instance of println, not on the second instance. This is because we haven't actually changed the value of s3: Excellent! You are now well-equipped, as long as you have the Java documentation handy, to call all sorts of String methods. Escape sequences If you spend a lot of time working with strings though, I anticipate that you're going to run into a common problem. Let's take a look at it quickly. I'm just going to write a brand new program here. I'm going to declare a string, then I'm going to have our program print the string to the screen. But the value I'm going to assign to this string is going to be a little tricky. I'd like our program to print out The program says: "Hello World" (I'd like Hello World to be surrounded by double quotation marks): The problem here is that putting double quotation marks inside a string literal confuses Java as shown in the preceding screenshot. When it reads through our program, the first full string it sees is "The program says:" that tells Java that we've ended our string. This is, of course, not what we want. [ 61 ] Understanding Typed Variables Fortunately, there's a system in place for us to tell Java that we'd like a character to be treated as a character literal instead of the special functionality that it might otherwise have. To do this, we put a backslash in front of the character. This is known as an escape sequence: String s= "The program says: \"Hello World\""; System.out.println(s); Now, when Java reads through this string, it will read The program says: and then see the backslash and know how to treat our double quotation marks as double quotation marks, the character, instead of the double quotation marks that surround a string. When we run our program, we will not see backslashes; they themselves are special characters: If we do want to see a backslash in our string, we need to preface it by a backslash of its own: String s= "The program says: \\ \"Hello World\""; System.out.println(s); That's strings 101! [ 62 ] Understanding Typed Variables Summary In this chapter, we explained what variables are and how important they are for creating better programs. We looked at some of the primitive data types of Java, namely int, long, float, char, and double, in detail. We also saw the String class and two of its manipulation methods. In the next chapter, we'll look at branching statements in Java. [ 63 ] 3 Branching Programs that perform the same action every time they run are all well and good, but the most interesting computer programs do something a little different each time they run, whether it's because they have differing input or even because a user is actively interacting with them. With this, let's kick-start this chapter by understanding conditional statements, then we will further explore how Java handles complicated conditional statements, modify the control flow of our program, and study loops functionality. Specifically, we'll cover the following topics in this chapter: Understanding if statements Complex conditionals The switch, case, and break statements The while and do...while loops The for loop Branching Understanding if statements Today, we're going to explore the very basic if and else conditional statements. To understand this further, refer to the following bullet list: 1. Let's create a new Java project in NetBeans. I'm going to call mine ConditionalStatements, and I will allow NetBeans to create the main class for me; refer to the following screenshot: To keep things clean, we can get rid of all our comments; now we're good to go. To allow us to write more interesting programs, we're going to quickly learn how to do some basic user input in Java. At this point in time, you don't have the knowledge base to fully grasp the intricacies of what we're about to do, but you may have a basic understanding of what's going on and you can certainly repeat the process on your own in future. While writing to this InputStream/Console window is kind of a simple fire-andforget process, reading input in Java can be a little more complicated: [ 65 ] Branching 2. User input is put into a buffer that our program accesses when it's prompted to; therefore, we need to declare a variable that will allow us to access this buffer when we need to get some new user input. For this, we're going to use the Scanner class. Let's call our new instance reader. NetBeans yells at us because Scanner lives in the java.util package, which we need to access explicitly. We can always import the java.util package: package conditionalstatements; public class ConditionalStatements { public static void main(String[] args) { java.util.Scanner reader; } } 3. Here's the part where you kind of have to take a leap of faith and jump a little bit ahead of what you're really ready to completely and totally understand. We need to assign a value to this reader variable of the Scanner type so that it links with the InputStream window, where our user will be entering their input. To do this, we're going to set its value to the value of a brand new Scanner() object, but this Scanner object is going to be created with a type argument, that is, (System.in), which happens to be the link to the InputStream our users will be using: package conditionalstatements; import java.util.*; public class ConditionalStatements { [ 66 ] Branching public static void main(String[] args) { Scanner reader = new Scanner(System.in); } } The following is the output of the preceding code: 4. Like I said, this is some heavy hitting stuff and you certainly shouldn't expect to understand how this works at a lower level right now. For now, know that reader is connected with our InputStream window, and our Scanner object has the next() function that allows us to access the input that the user has just entered into the stream. Like most functions, this function simply returns this input, so we're going to have to create a string to store this input. 5. Once we've done this, we can use our System.out.println() function to print the input value back to the console: package conditionalstatements; import java.util.*; public class ConditionalStatements { public static void main(String[] args) { Scanner reader = new Scanner(System.in); String input = reader.next(); System.out.println(input); } } 6. When we run our program, nothing appears to happen, but in actuality, our console here is waiting for some user input. Now, when we type our input into this console and hit Enter key, it will echo right back at us: [ 67 ] Branching 7. We can make this a little nicer to use by having our program prompt the user for the input rather than simply waiting quietly: public static void main(String[] args) { Scanner reader = new Scanner(System.in); System.out.println("Input now: "); String input = reader.next(); System.out.println(input); } Conditional statements At the beginning of this chapter, I promised you'd learn about conditional statements, and we're about to do that now. But first, let's make a quick modification to the user input portion of our program. Rather than acquiring a string, it's going to be a lot easier if we learn conditional statements working with a user-provided integer value. So let's change the value or the type of our input variable to an int datatype; the reader.next() function returns a string, but there's a similar function called nextInt() that will return an integer: int input = reader.nextInt(); We're certainly not going to bother putting any error-handling mechanism in our very simple program. Know that if we accidentally provide this Java program with anything besides an integer, the program will crash. [ 68 ] Branching So what are conditional statements exactly? Well, conditional statements allow us to send our program down different paths, executing different lines of code depending on whether or not something is true or not. In this chapter, we'll use conditional statements to print different responses to our user depending on the value of the input they give us. Specifically, we'll let them know whether the value they've given us is less than, greater than, or equal to the number 10. To start this process off, let's set up our output cases. If our user provides us with input that is greater than 10, we print out MORE. If the user provides us with input that happens to be less than 10, we print out LESS. Of course, if we run this program right now, it will simply print out MORE or LESS, both the lines. What we need to do is use conditional statements to make sure that only one of these two lines executes in any program run, and that the proper line executes of course. You may have noticed that the default project NetBeans created for us divides our code into segments that have curly brackets around them. We can further divide our code into segments using brackets of our own. Convention dictates that once we've created a new set of brackets, a new segment of code, we need to then add a tab before everything between the brackets to make our program more readable. Using if statements Once we've sectioned off our two system.out.println statements, we're now ready to provide cases that must be true if these statements are to run. To do this, we preface our new sections with the Java if statement, where if is a Java keyword and it's followed by two parentheses between which we put the statement to be evaluated. If Java determines that the statement we write between the parentheses is true, the code in the following brackets will execute. If Java determines that the statement is false, the code in the brackets will be completely skipped. Essentially, we're going to give this if statement two pieces of input. We're going to give it the variable input, which if you remember contains the integer value that we got from the user, and we're going to give it the explicit value 10, which is what we're comparing it to. Java understands the greater than (>) and the less than ( 10), then the System.out.println command (as seen in the following screenshot) will only run if the user has provided a value that's greater than 10: if(input > 10) { System.out.println("MORE!"); } { System.out.println("LESS!"); } [ 69 ] Branching Now, we need to provide an if statement to make sure that our program doesn't always print out LESS anyway. We could use the less than operator to ask our program to print out LESS whenever the user provides input that is less than 10. This would be good in almost all cases, but if our user provides the input value 10, our program would print out nothing. To fix this, we can use the less than or equal to operator to ensure that our program always responds to the user input: package conditionalstatements; import java.util.*; public class ConditionalStatements { public static void main(String[] args) { Scanner reader = new Scanner(System.in); System.out.println("Input now: "); int input = reader.nextInt(); if(input > 10) { System.out.println("MORE!"); } if(input 10) { System.out.println("MORE!"); } else { System.out.println("LESS"); } } } [ 71 ] Branching If we run this program, we will get the same results as we did before with one less bit of logic code to write: Let's end this topic with a brief run-through of what other operators we can use in our if statements, then we'll take a look at what to do if we need to compare items that are not primitives. In addition to the greater than and less than operators, we can also make use of the equality operator (==), which is true if the items on both the sides have the same value. Make sure when you use the equality operator, do not accidently use the assignment operator (=) instead: if(input == 10) In some instances, your program won't compile, but at other times it will compile and you'll get very weird results. If you'd like to use the opposite of the equality operator, you can use not equals (!=), which returns true if the two items do not have the same value: if(input != 10) It's important that we do not attempt to use these equality operators when comparing instances of a class. We should only use them when we're working with primitives. To show this, let's modify our program so that we could take String as user input. We'll see whether String is equivalent to the secret password code: [ 72 ] Branching If it is, it will print out YES; if not, it will print out NO. Now, NetBeans gives us a warning (as shown in the preceding screenshot); in fact, if we attempt to compare strings with some different operators, NetBeans would let us know that our program is probably not even going to compile. That's because Java does not expect us to compare instances of a class with these operators. Instead, classes should expose functions that allow us to compare them logically. Almost every object in Java has a few functions for this purpose. One of the most common ones is the equals() function that takes an object of the same type and lets us know whether they're equivalent. This function returns what's called a Boolean type, which is a primitive type of its own that can have a value of either true or false. Our if statement understands how to evaluate this Boolean type: import java.util.*; public class ConditionalStatements { public static void main(String[] args) { Scanner reader = new Scanner(System.in); System.out.println("Input now: "); String input = reader.next(); if(input.equals("password")) { System.out.println("YES"); } else { System.out.println("NO"); } } } Let's run our program quickly and start by inputting a bad string, then let's input password to see our program work: [ 73 ] Branching That's the basics of if-else statements. I would encourage you now to play with some of the comparison operators that we looked at and try nesting if...else statements within each other. As a very last note, you may sometimes see if statements without their following brackets. This is a valid syntax and is basically the equivalent of putting the entire statement on one line. Complex conditionals To begin with, let's write a very simple Java program. We'll start by importing java.util so that we can get some user input via a Scanner object, and we'll link this Scanner object with the System.in input string so we can use it in the console window. Once we've done this, we're going to need to get some input from the user and store it, so let's create a new string and assign its value to whatever the user gives us. To keep things interesting, let's give ourselves two more String variables to work with. We'll call them sOne and sTwo; we'll assign the value of our first string variable to abc and the value of our second string variable to just z: package complexconditionals; import java.util.*; public class ComplexConditionals { public static void main(String[] args) { Scanner reader = new Scanner (System.in); String input = reader.next(); String sOne = "abc"; String sTwo = "z"; } } Because this topic is about conditional statements, we're probably going to need one of those, so let's create an if...else block. This is where we'll evaluate our conditional statement. We'll set ourselves up some output so we can see what's going on. If our conditional statement evaluates to true and we enter the following portion of the block, we'll simply print out TRUE: if() { System.out.println("TRUE"); [ 74 ] Branching } else { } If the conditional statement evaluates to false and we skip the previous if portion of the block and instead enter the else portion, we'll print out FALSE: if() { System.out.println("TRUE"); } else { System.out.println("FALSE"); } The contains function Now it's probably time to write our conditional statement. Let me introduce you to a new string function called the contains function: if(input.contains()) The contains function takes as input a sequence of characters of which a string qualifies. As output, it gives us a Boolean value, which means it will either output TRUE or FALSE. So our if statement should understand the result of this function and evaluate to the same. To test our program, let's start by simply going through the following process. We'll provide our contains function with the value stored in the sOne string which is abc: package complexconditionals; import java.util.*; public class ComplexConditionals { public static void main(String[] args) { Scanner reader = new Scanner (System.in); String input = reader.next(); String sOne = "abc"; String sTwo = "z"; if(input.contains(sOne)) { System.out.println("TRUE"); } else [ 75 ] Branching { System.out.println("FALSE"); } } } So, if we run our program and provide it with abcdefg which contains within it the abc string, we get the TRUE result. This is because input.contains evaluates to true and we enter the if portion of our if...else block: If we were to run and provide some gibberish that does not contain the abc string, we could instead enter the else statement of the block and return FALSE: Nothing too crazy there. But, let's say we want to make our program a little more complicated. Let's look at this in the next section. [ 76 ] Branching Complex conditional statements What if we want to check and see whether our input string contains both the strings, namely sOne and sTwo? There's a couple of ways to do this, and we'll take a look at some others. But probably, the simplest way for our purposes is to use a complex conditional on the if(input.contains(sOne)) line. Java allows us to evaluate multiple true or false statements, or Boolean objects, at once using the && or the | conditional operator. The && operator gives us a true result when all conditionals compared with the && operator have evaluated to true. The | operator gives us a true result when any of the conditionals compared with the | operator evaluate to true. In our case, we want to know whether our input string contains both the contents of sOne and sTwo, so we're going to use the && operator. This operator works by simply providing two conditional statements on either side of it. So, we're going to run our input.contains function on both sOne and sTwo. If both these functions on either side of the && operator, which is expressed as (if(input.contains (sOne) && input.contains (sTwo)), evaluate to true, our conditional statement will be true as well: package complexconditionals; import java.util.*; public class ComplexConditionals { public static void main(String[] args) { Scanner reader = new Scanner (System.in); String input = reader.next(); String sOne = "abc"; String sTwo = "z"; if(input.contains(sOne)) { System.out.println("TRUE"); } else { System.out.println("FALSE"); } } } [ 77 ] Branching Let's run our program. The abcz string should evaluate to true in both cases, and when we press Enter key, we see that this is in fact the case: If we were to provide only the valid string z, we would get a false result because our && operator would evaluate false and true, which evaluates to false. If we were to use the | operator, this would be the string: if(input.contains(sOne) || input.contains(sTwo)) The following is the output of the preceding code: This would in fact give us a true result because we only need one of these functions to return true now. Boolean logic can get pretty crazy pretty quick. For example, we can put the && false statement at the end of our Boolean condition, namely if(input.contains(sOne) || input.contains(sTwo) && false). The true and false code terms in Java are keywords; in fact, they're explicit values, just like a number or a single character would be. The true keyword evaluates to true and the false keyword evaluates to false. Any single conditional statement that ends with false will always evaluate as a whole to false: if(input.contains(sOne) && false) [ 78 ] Branching Curiously though, if we return to our previous original statement and run the following program providing it with the most valid possible input, we're going to get the true result: package complexconditionals; import java.util.*; public class ComplexConditionals { public static void main(String[] args) { Scanner reader = new Scanner(System.in); String input = reader.next(); String sOne = "abc"; String sTwo = "z"; if(input.contains(sOne) || input.contains(sTwo) && false) { System.out.println("TRUE"); } else { System.out.println("FALSE"); } } } Following is the output of the preceding code: That's interesting because if Java had chosen to evaluate the if(input.contains(sOne) || input.contains(sTwo)) statement first and then the && false statement, we would have gotten a false result; instead, Java seems to have chosen to evaluate the (input.contains(sTwo) && false) statement first and then the || statement, that is, (input.contains(sOne) ||). This can make things confusing pretty quick. [ 79 ] Branching Fortunately, just like in algebra, we can ask Java to do operations in a specific order. We do this by surrounding the blocks of our code with parentheses. Blocks of code within parentheses will evaluate before Java leaves the parentheses to evaluate against something else: if((input.contains(sOne) || input.contains(sTwo)) && false) So, after we've surrounded our || statement with parentheses, we'll compute the || statement and then end that result with false: package complexconditionals; import java.util.*; public class ComplexConditionals { public static void main(String[] args) { Scanner reader = new Scanner(System.in); String input = reader.next(); String sOne = "abc"; String sTwo = "z"; if((input.contains(sOne) || input.contains(sTwo)) && false) { System.out.println("TRUE"); } else { System.out.println("FALSE"); } } } We will now see that our preceding program always evaluates to false here: So complex conditionals can get pretty complicated pretty quick. If we come across something like this if statement in code, especially if it's code that we haven't written, it can take us a good while to figure out exactly what's going on. [ 80 ] Branching The boolean variable To help us with what we discussed in the preceding section, we have the boolean variable: boolean bool = true; In the preceding line of code, boolean is a primitive type in Java, and a variable of the boolean type can have only one of the two values: it can be either true or false. We can set the value of our Boolean variables to be any conditional statement. So, if we wanted to simplify how our code looks in the actual if statement, we could go ahead and store these Boolean values: boolean bool1 = input.contains(sOne); boolean bool2 = input.contains(sTwo); We need to do this before we actually evaluate the if statement, keeping everything much more compact and readable: if((bool1 || bool2) && false) Remember that the name of the game is keeping our code as simple and readable as possible. A really long conditional might feel great to write, but oftentimes, there's a more elegant solution. That's the nuts and bolts of complex conditionals in Java. Switch, case, and break In this section, we're going to take a look at the switch statement, which is another way that we can modify the control flow of our program. To begin, let's create a new project in NetBeans. At my end at least, I'm going to get rid of all these comments. To demonstrate the power of the switch statement, we're going to start by writing a program using only if blocks, then we'll convert the program to one that uses switch statements. The following are the steps for the program that uses only if blocks: 1. To begin, let's simply declare a variable x, (int x =1;), and here is our goal: If the value of x is 1, 2, or 3, we'd like to print out the responses RED, BLUE, or GREEN, respectively. If x is not one of those numbers, we'll just print out a default response. [ 81 ] Branching 2. Doing this with if blocks is pretty straightforward, if not a little tedious: if(x == 1) { System.out.println("RED") } Then, we'll pretty much just copy and paste this block of code and modify it for the blue and green cases: int x=1; if(x==1) { System.out.println("RED"); } if(x==2) { System.out.println("BLUE"); } if(x==3) { System.out.println("GREEN"); } 3. For our default case, we simply want to check that x is not equal to 1, x is not equal to 2, and x is not equal to 3: if((x != 1) && (x != 2) && (x != 3)) { System.out.println("NONE"); } Let's give our program a very quick run-through: package switcher; public class Switcher { public static void main(String[] args) { int x=1; if(x==1) { System.out.println("RED"); } if(x==2) { System.out.println("BLUE"); } if(x==3) [ 82 ] Branching { System.out.println("GREEN"); } } } The following is the screenshot of the expected results: This is a simplified version of something we might logically find ourselves doing in the course of writing a bigger program. While we put together this at a pretty decent clip, it's easy to see how this problem would become extremely unwieldy if we were dealing with many possible cases of x. And, it's also pretty difficult for someone to read and figure out what's going on here. The solution, as you've probably guessed, is to use a switch statement to control our program's flow instead. Program using switch, case, and break When we want to execute different lines or blocks of code depending on the value of an individual variable, the switch statement is extremely effective. Now let's rewrite our series of if blocks using a switch statement instead. The syntax is explained in the following steps: 1. We first declare that we're going to use a switch statement, switch being a reserved keyword in Java. Then, we provide the name of the variable that we'd like the switch statement to act on, in this case x because we're going to execute different blocks of code depending on the value of x: package switcher; public class Switcher { public static void main(String[] args) { int x=1; switch(x) { [ 83 ] Branching } } } Then, just like using an if or else statement, we're going to create a new segment of code using two brackets. 2. Now, instead of creating a series of unwieldy if blocks, we create separate blocks in our switch statements using the case keyword. After each case keyword, we give a prescribed value, and the following code will execute if the value of x matches with the value of the case keyword. So, just like when we were doing our if blocks, if the value of x is 1, we'd like to print out RED. Writing separate cases for each of the possible values now becomes much cleaner and easier to read. 3. The switch statements also have a special case, the default case, which we pretty much always put at the end of a switch statement. This case will only execute if none of the other cases have executed, and it means that we don't have to write that complicated Boolean logic for our last if block: package switcher; public class Switcher { public static void main(String[] args) { int x=7; switch(x) { case 1: case 5: case 7: System.out.println("RED"); case 2: System.out.println("BLUE"); case 3: System.out.println("GREEN"); default: System.out.println("NONE"); } } } [ 84 ] Branching If we run the preceding program, we're actually going to see every possible output execute. That's because we've forgotten to do something very important: The switch statements allow us to create complicated logic trees because once a case starts executing, it will continue to execute even through the next cases in the queue. Because we're writing a very simple program where we only want a single case to execute, we need to explicitly end the execution once we enter a case and finish the code. 4. We can do that with the break keyword, which exists on a line of code all by itself and simply steps us up and out of the case that we're in: package switcher; public class Switcher { public static void main(String[] args) { int x=1; switch(x) { case 1: System.out.println("RED"); break; case 2: System.out.println("BLUE"); break; case 3: System.out.println("GREEN"); break; default: System.out.println("NONE"); } } } [ 85 ] Branching Now if we run our program, we'll see the expected results: 5. In addition to falling through from one case to another, we can increase the complexity and the power of our switch statements by adding multiple cases to a single line. Because the cases freely fall through to each other, doing something like case 1: case 5: case; means that the following block of code will execute if we provide one of these numbers: 1, 5, or 7. So there's the quick and easy way of switch statements: package switcher; public class Switcher { public static void main(String[] args) { int x=7; switch(x) { case 1: case 5: case 7: System.out.println("RED"); break; case 2: System.out.println("BLUE"); break; case 3: System.out.println("GREEN"); break; default: System.out.println("NONE"); } } } [ 86 ] Branching The following is the output of the preceding code: Switch statements essentially compare the variable or explicit value we're switching and the cases using the equality (==) operator. If elements cannot be compared with the equality operator, the switch statement will not work properly. As of Java SE v7, you can compare strings with the equality operator so you can use them in switch statements. This was not always the case, and it's still a good idea to avoid using strings with the equality operator in your switch statements. This is because it destroys the backwards compatibility of the code you're writing. While and do...while loops Welcome to the introductory lesson on loops. At the end of this section, we'll have command over Java's while and do...while loops. I'm pretty excited about this because loops allow us to execute a block of Java code over and over again as many times as we see fit. This is a pretty cool step in our learning process because the ability to perform small tasks many times in rapid succession is one of the things that makes computers better at certain tasks than humans are: 1. To begin this topic, let's create a new NetBeans project, enter the main method, and simply declare an integer and give it a value. We can choose any positive value. We're going to ask our program to print out the phrase Hello World a number of times equal to the value of our integer. [ 87 ] Branching 2. To do this, we'll employ a while loop. The while loop syntax looks a lot like we're writing an if statement. We begin with the reserved while keyword and follow it by two parentheses; inside these, we're eventually going to place a conditional statement. Just like it was an if statement, the following block of code will only execute if our program reaches our while loop and evaluates its conditional statement to true: package introtoloops; public class IntroToLoops { public static void main(String[] args) { int i=5; while () { } } } What separates while loops from if statements, however, is that when the end of the while loop's block of code is reached, our program will basically jump back and execute this line of code again, evaluating the conditional statement and reentering the while loop's block of code if the conditional statement is still true. Let's start off by setting up the logic for our while loop. We have the number of times we would like our loop to execute stored in the value of the integer i, but we're going to need a way to communicate this to our loop. Well, any loop that is not going to run infinite times is going to need to make some control flow changes within the content of the loop. In our case, let's change the state of our program each time the loop runs by decreasing the value of i so that when i reaches 0, we would have ran the loop five times. 3. If that's the case, it means that we only want our loop to execute when the value of i is greater than 0. Let's pause and quickly take a look at this line of code. Here i = i -1 is a perfectly valid statement, but there's a shortcut we can use that's a little faster and easier to read. We can use i-- to decrement the value of an integer variable by one. Once we've set this up, the only thing left to do is place the functional code inside of our loop; that's simply a println statement that says Hello world: package introtoloops; public class IntroToLoops { public static void main(String[] args) { [ 88 ] Branching int i=5; while (i>0) { System.out.println("Hello world"); i--; } } } 4. Now, let's run our program and see what happens: There we go, five Hello world instances printed to our console window, just as we intended. While loops Often, we allow small programs, such as the ones we're writing here, to end when there's simply no more code for them to execute. However, while we're working with loops, we're probably going to make the mistake of accidentally creating an infinite while loop and running a program that has no end: package introtoloops; public class IntroToLoops { public static void main(String[] args) { int i=5; while (i>0) { System.out.println("Hello world"); } } } [ 89 ] Branching When this happens, we'll need to turn our program off manually. In NetBeans, there's a handy little feature called Stop at the left-hand side of the output window: If we're running a program through a Command Prompt, Ctrl + C is the common command to cancel the execution of a program. Now that we have a grasp of the basic while loop syntax, let's try something a little more complicated and much more dynamic: 1. The program I have in mind is going to need some user input, so let's import java.util and set up a new Scanner object: public class IntroToLoops { public static void main(String[] args) { Scanner reader = new Scanner(System.in); 2. Rather than gathering user input right away though, we're going to collect new user input every time our while loop successfully executes: while(i > 0) { reader.nextLine(); System.out.println("Hello world"); } 3. Each time we gather this input, we're going to need somewhere to store it, so let's create a new string whose purpose will be to store the value of the newly acquired input: public class IntroToloops { public static void main(String[] args) { Scanner reader = new Scanner(System.in); String input; int i=5; while(i>0) { input= reader.nextLine(); System.out.println("Hello world"); [ 90 ] Branching } } } This input variable's value will change a number of times throughout the execution of our program because at the beginning of every while loop, we'll be assigning it a new value. If we were to simply execute this program, it wouldn't be very interesting for us, the user, because the old values of our input string would constantly be lost when we assign it a new value. 4. So, let's create another string whose purpose is to store all the concatenated values that we've gotten from the user. Then, at the end of our program, we'll print out the value of this string so that the user can see we've been storing their input: public class IntroToloops { public static void main(String[] args) { Scanner reader = new Scanner(System.in); String input; String all = ""; int i=5; while(i>0) { input = reader.nextLine(); } System.out.println(all); } } 5. Add the value of the input to the all string on the line illustrated here: while(i>0) { input = reader.nextLine(); all = } [ 91 ] Branching There's a couple of things we can do. We can add strings with the addition operator just fine. So the all = all + input statement, with all and input being strings, and the plus sign are perfectly valid. However, when we add something to itself and work with a primitive or a type that can act like a primitive as a string can, we can also use the += operator, which performs the same function. Also, we cannot forget to reimplement the decrement of the integer value i so that our program doesn't run infinitely: package introtoloops; import java.util.*; public class IntroToLoops { public static void main(String[] args) { Scanner reader = new Scanner(System.in); String input; String all = ""; int i=5; while (i>0) { input = reader.nextLine(); all += input; i--; } System.out.println(all); } } Now, if we run this program and provide five strings of input, we get the output as shown in the following screenshot: We'll see them all spit back out at us as expected, which is pretty cool, but I've got bigger plans for this program. [ 92 ] Branching Actually, if we'd only wanted to write the program we have right here, a for loop, which we'll learn later, might have been completely appropriate. But for what we're about to do, while and do...while loops are pretty necessary. What I'd like to do is get rid of our counting variable in this program. Instead, we're going to allow the user to tell us when to stop executing our program. When the user sets the value of the input to the STOP string, in all capital letters, we'll quit executing our while loop and print out all the strings they've given us so far. So, we only want this while loop to run while the value of the input is not the STOP value. You'll notice that we'll get a precompiled error as shown in the following screenshot: We'll get a full-on compiler error if we do attempt to run our program. That's because our program knows that when we attempt to execute this conditional statement for the first time, the value of the input will not have been set. Even though the nonexistent value of the input is not equivalent to STOP, it's a very bad form. In the case of a string here, which is not a primitive, it is not possible for our computer to access any of its methods before it's given any value at all. One inelegant solution here would be to give the input a start value, like we did with all, but there's a better way. Once our loop has executed for the first time, we know that the input will have a proper value given by the user, which may or may not be STOP. The do...while loops What if instead of checking our conditional at the beginning of our loop, we check it at the end? This is in fact an option. The do...while loops operate just like while loops, but the first time they run, they won't check to see whether a conditional is true; they'll simply run through and check their conditional statement at the end. We do need to put a semicolon at the end of a conditional statement on the back of the do...while loop. I just mentioned this because I always forget it. Now, if we run our program, we can give any number of strings and then type in the STOP string to see everything we've typed so far and printed out to the screen: public static void main(String[] args) { Scanner reader = new Scanner(System.in); [ 93 ] Branching String input; String all = ""; int i=5; do { input = reader.nextLine(); all += input; i--; } while(!input.equals("STOP")); System.out.println(all); } The following is the output of the preceding code: As a last little note, with just about anything followed by its own code block, you'll see a syntax like this, where you will have a keyword and may be a conditional statement and then brackets on the following lines; alternatively, you might see the brackets start on the same line as the keyword and the conditional statement. Both the methods are completely valid, and in fact, the brackets starting on the same line as the keyword is probably quickly becoming more common. I encourage you to play around with the program we've written. Try executing loops that you think will push the boundaries of how much information a string can hold, or play around with loops that present an enormous amount of information to the screen. This is stuff that computers do that we simply couldn't with pencil and paper, so it's pretty neat. [ 94 ] Branching For loops In this section, we're going to take a quick look at for loops. We use for loops to solve a common problem in Java in a very semantically elegant manner. These loops are appropriate when we need to iterate a variable to count how many times we've looped. To start off, I've written a very basic program using a while loop; it prints the values 1 through 100 to the window on our screen. Once you've hashed out in your mind how this while loop is working, we'll write the same loop with a for loop so that we could see how the for loop is more elegant in this particular instance. Let's comment out our while loop so that we can still see it as shown in the following screenshot without having it execute any of its code and begin writing our for loop instead : The basic syntax of a for loop looks very similar to that of a while loop. We have the reserved keyword, two parentheses in which we'll put some information that the loop needs, and the code block that we're going to loop through. Unlike a while loop, which just takes a conditional statement between these parentheses, we're going to provide our for loop with a bunch of information. Because the for loop is designed to handle a specific case, once we give it all this information, it will know exactly what to do with it. This alleviates the need for us to handle code outside of the loop and manually increment or decrement it inside of the loop. It keeps the functional bit of our code, our println statement, and in more complicated programs, the more complicated information that may be inside the for loop, more isolated. [ 95 ] Branching Our typical for loop takes three pieces of input. These are as follows: 1. First, we need to declare the variable that we're going to be incrementing or decrementing to count the number of times we've looped. In this case, we'll use an integer i and give it a initial value of 1. We follow this initial statement with a semicolon. This is not a function call; it's special syntax for the for loop. 2. The second piece of information the special syntax requires is the conditional statement that we need to evaluate each time we're going to restart the loop. If this conditional statement is ever not true, then our for loop ends and we continue resuming our code after the for loop block. In this case, our conditional statement will be the same as it was for the while loop. We want our last iteration of the for loop to be when i is equal to 100, that is, when we print out 100. Once i is no longer less than or equal to 100, it's time to exit our for loop. 3. Just as the first piece of information we specially gave the for loop saved us from having to handle a variable outside the scope of our loop, the last piece of information we will give the for loop takes the place of us having to manually increment or decrement our counter inside the scope of the loop. This is the special modification code, and whatever we provide the for loop here will run at the end of every loop. In this case, we'd just like to increment the value of i at the end of each looping through. I think you'll agree that this program is much cleaner than our while loop: package forloops; public class Forloops { public static void main(String[] args) { /* int i=1; while(i |
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