Chapter 1 Introduction PDF

CHAPTER 1 INTRODUCTION CHAPTER GOALS To learn about computers and programming To compile and run your first Java program © JanPi...

CHAPTER 1 INTRODUCTION CHAPTER GOALS To learn about computers and programming To compile and run your first Java program © JanPietruszka/iStockphoto. To recognize compile-time and run-time errors To describe an algorithm with pseudocode CHAPTER CONTENTS 1.1 COMPUTER PROGRAMS 2 1.6 ERRORS 13 CE 2 Misspelling Words 14 1.2 THE ANATOMY OF A COMPUTER 3 C&S Computers Are Everywhere 5 1.7 PROBLEM SOLVING: ALGORITHM DESIGN 15 1.3 THE JAVA PROGRAMMING HT 1 Describing an Algorithm with LANGUAGE 5 Pseudocode 18 1.4 BECOMING FAMILIAR WITH YOUR WE1 Writing an Algorithm for Tiling a Floor 20 PROGRAMMING ENVIRONMENT 7 PT 1 Backup Copies 10 1.5 ANALYZING YOUR FIRST PROGRAM 11 SYN Java Program 12 CE 1 Omitting Semicolons 13 1 bjeo_ch01p.indd 1 11/21/18 5:11 PM  Just as you gather tools, study a project, and make a plan for tackling it, in this chapter you will gather up the basics you need to start learning to program. After a brief introduction to computer hardware, software, and programming in general, you will learn how to write and run your first Java program. You will also learn how to diagnose and fix programming errors, and how to use pseudocode to describe an algorithm—a step-by-step description of how to solve a problem—as you plan your computer programs. © JanPietruszka/iStockphoto. 1.1 Computer Programs Computers You have probably used a computer for work or fun. Many people use computers for execute very basic everyday tasks such as electronic banking or writing a term paper. Computers are instructions in good for such tasks. They can handle repetitive chores, such as totaling up numbers rapid succession. or placing words on a page, without getting bored or exhausted. The flexibility of a computer is quite an amazing phenomenon. The same machine can balance your checkbook, lay out your term paper, and play a game. In contrast, other machines carry out a much narrower range of tasks; a car drives and a toaster toasts. Computers can carry out a wide range of tasks because they execute different programs, each of which directs the computer to work on a specific task. A computer program The computer itself is a machine that stores data (numbers, words, pictures), inter- is a sequence acts with devices (the monitor, the sound system, the printer), and executes programs. of instructions A computer program tells a computer, in minute detail, the sequence of steps that and decisions. are needed to fulfill a task. The physical computer and peripheral devices are collec- tively called the hardware. The programs the computer executes are called the software. Today’s computer programs are so sophisticated that it is hard to believe that they are composed of extremely primitive instructions. A typical instruction may be one of the following: Put a red dot at a given screen position. Add up two numbers. If this value is negative, continue the program at a certain instruction. The computer user has the illusion of smooth interaction because a program contains a huge number of such instructions, and because the computer can execute them at great speed. Programming is the The act of designing and implementing computer programs is called program- act of designing ming. In this book, you will learn how to program a computer—that is, how to direct and implementing the computer to execute tasks. computer programs. To write a computer game with motion and sound effects or a word processor that supports fancy fonts and pictures is a complex task that requires a team of many highly-skilled programmers. Your first programming efforts will be more mundane. The concepts and skills you learn in this book form an important foundation, and you should not be disappointed if your first programs do not rival the sophisticated software that is familiar to you. Actually, you will find that there is an immense thrill even in simple programming tasks. It is an amazing experience to see the computer 2 bjeo_ch01p.indd 2 11/21/18 5:11 PM  1.2 The Anatomy of a Computer 3 precisely and quickly carry out a task that would take you hours of drudgery, to make small changes in a program that lead to immediate improvements, and to see the computer become an extension of your mental powers. 1.2 The Anatomy of a Computer To understand the programming process, you need to have a rudimentary under- standing of the building blocks that make up a computer. We will look at a personal computer. Larger computers have faster, larger, or more powerful components, but they have fundamentally the same design. At the heart of the computer lies the central processing unit (CPU) (see Figure 1). The inside wiring of the CPU is enormously complicated. For example, the Intel Core processor (a popular CPU for personal computers at the time of this writing) is composed of several hundred million structural elements, called transistors. The central The CPU performs program control and data processing unit (CPU) processing. That is, the CPU locates and exe- performs program cutes the program instructions; it carries out control and data processing. arithmetic operations such as addition, subtrac- tion, multiplication, and division; it fetches data from external memory or devices and places © Amorphis/iStockphoto. processed data into storage. Storage devices There are two kinds of storage. Primary stor- Figure 1 Central Processing Unit include memory and age, or memory, is made from electronic circuits that can store data, provided they are secondary storage. supplied with electric power. Secondary storage, usually a hard disk (see Figure 2) or a solid-state drive, provides slower and less expensive storage that persists without electricity. A hard disk consists of rotating platters, which are coated with a magnetic © PhotoDisc, Inc./Getty Images, Inc. Figure 2 A Hard Disk bjeo_ch01p.indd 3 11/21/18 5:11 PM  4 Chapter 1 Introduction material. A solid-state drive uses electronic components that can retain information without power, and without moving parts. To interact with a human user, a computer requires peripheral devices. The com- puter transmits information (called output) to the user through a display screen, speakers, and printers. The user can enter information (called input) for the computer by using a keyboard or a pointing device such as a mouse. Some computers are self-contained units, whereas others are interconnected through networks. Through the network cabling, the computer can read data and programs from central storage locations or send data to other computers. To the user of a networked computer, it may not even be obvious which data reside on the com- puter itself and which are transmitted through the network. Figure 3 gives a schematic overview of the architecture of a personal computer. Program instructions and data (such as text, numbers, audio, or video) reside in sec- ondary storage or elsewhere on the network. When a program is started, its instruc- tions are brought into memory, where the CPU can read them. The CPU reads and executes one instruction at a time. As directed by these instructions, the CPU reads data, modifies it, and writes it back to memory or secondary storage. Some program instructions will cause the CPU to place dots on the display screen or printer or to vibrate the speaker. As these actions happen many times over and at great speed, the human user will perceive images and sound. Some program instructions read user input from the keyboard, mouse, touch sensor, or microphone. The program ana- lyzes the nature of these inputs and then executes the next appropriate instruction. Printer Mouse/Trackpad Disk Ports controller Secondary storage Keyboard CPU Monitor Microphone Memory Speakers Video camera Network controller Internet Figure 3 Schematic Design of a Personal Computer bjeo_ch01p.indd 4 11/21/18 5:11 PM  1.3 The Java Programming Language 5 Computing & Society 1.1 Computers Are Everywhere When computers were The advent of ubiqui- first invented in the tous computing changed © Media Bakery. 1940s, a computer filled an entire many aspects of our room. The photo below shows the lives. Factories used ENIAC (electronic numerical integrator to employ people to and computer), completed in 1946 at do repetitive assembly the University of Pennsylvania. The tasks that are today car- ENIAC was used by the military to ried out by computer- compute the trajectories of projectiles. controlled robots, oper- Nowadays, computing facilities of ated by a few people search engines, Internet shops, and who know how to social networks fill huge buildings work with those com- called data centers. At the other end of puters. Books, music, the spectrum, computers are all around and movies nowadays © Maurice Savage/Alamy Limited. us. Your cell phone has a computer are often consumed inside, as do many credit cards and fare on computers, and This transit card contains a computer. cards for public transit. A modern car computers are almost has several computers––to control the always involved in their production. now could not have been written with- engine, brakes, lights, and the radio. The book that you are reading right out computers. Knowing about computers and how to program them has become an essential skill in many careers. Engi- neers design computer-controlled cars and medical equipment that preserve lives. Computer scientists develop programs that help people come together to support social causes. For example, activists used social net- works to share videos showing abuse by repressive regimes, and this infor- mation was instrumental in changing public opinion. As computers, large and small, become ever more embedded in our everyday lives, it is increasingly important for everyone to understand how they work, and how to work with them. As you use this book to learn how to program a computer, you will develop a good understanding of com- puting fundamentals that will make © UPPA/Photoshot. you a more informed citizen and, per- haps, a computing professional. The ENIAC 1.3 The Java Programming Language In order to write a computer program, you need to provide a sequence of instructions that the CPU can execute. A computer program consists of a large number of simple CPU instructions, and it is tedious and error-prone to specify them one by one. For that reason, high-level programming languages have been created. In a high-level bjeo_ch01p.indd 5 11/29/18 3:53 PM  6 Chapter 1 Introduction language, you specify the actions that your program should carry out. A compiler translates the high-level instructions into the more detailed instructions (called machine code) required by the CPU. Many different programming languages have been designed for different purposes. In 1991, a group led by James Gosling and Patrick Naughton at Sun Microsystems designed a program- ming language, code-named “Green”, for use in con- sumer devices, such as intelligent television “set-top” boxes. The language was designed to be simple, secure, and usable for many different processor types. No cus- tomer was ever found for this technology. Java was originally Gosling recounts that in 1994 the team realized, designed for “We could write a really cool browser. It was one of the programming few things in the client/server mainstream that needed consumer devices, but it was first some of the weird things we’d done: architecture neu- © James Sullivan/Getty Images. successfully used tral, real-time, reliable, secure.” Java was introduced to to write Internet an enthusiastic crowd at the SunWorld exhibition in James Gosling applets. 1995, together with a browser that ran applets—Java code that can be located anywhere on the Internet. The figure at right shows a typical example of an applet. Since then, Java has grown at a phenomenal rate. Programmers have embraced the language because it is easier to use than its closest rival, C++. In addi- tion, Java has a rich library that makes it possible to write portable programs that can bypass pro- prietary operating systems—a feature that was eagerly sought by those who wanted to be inde- pendent of those proprietary systems and was bit- terly fought by their vendors. A “micro edition” An Applet for Visualizing Molecules and an “enterprise edition” of the Java library allow Java programmers to target hardware ranging from smart cards to the largest Internet servers. Table 1 Java Versions (since Version 1.0 in 1996) Version Year Important New Features Version Year Important New Features 1.1 1997 Inner classes 6 2006 Library improvements 1.2 1998 Swing, Collections framework 7 2011 Small language changes and library improvements 1.3 2000 Performance enhancements 8 2014 Function expressions, streams, new date/time library 1.4 2002 Assertions, XML support 9 2017 Modules 5 2004 Generic classes, enhanced for 10, 11 2018 Versions with incremental loop, auto-boxing, improvements are released enumerations, annotations every six months bjeo_ch01p.indd 6 11/21/18 5:11 PM  1.4 Becoming Familiar with Your Programming Environment 7 Java was designed to Because Java was designed for the Internet, it has two attributes that make it very be safe and portable, suitable for beginners: safety and portability. benefiting both Java was designed so that anyone can execute programs in their browser without Internet users and students. fear. The safety features of the Java language ensure that a program is terminated if it tries to do something unsafe. Having a safe environment is also helpful for anyone learning Java. When you make an error that results in unsafe behavior, your program is terminated and you receive an accurate error report. Java programs The other benefit of Java is portability. The same Java program will run, without are distributed as change, on Windows, UNIX, Linux, or Macintosh. In order to achieve portability, instructions for a the Java compiler does not translate Java programs directly into CPU instructions. virtual machine, making them Instead, compiled Java programs contain instructions for the Java virtual machine, a platform-independent. program that simulates a real CPU. Portability is another benefit for the beginning student. You do not have to learn how to write programs for different platforms. At this time, Java is firmly established as one of the most important languages for general-purpose programming as well as for computer science instruction. However, although Java is a good language for beginners, it is not perfect, for three reasons. Because Java was not specifically designed for students, no thought was given to making it really simple to write basic programs. A certain amount of technical machin- ery is necessary to write even the simplest programs. This is not a problem for profes- sional programmers, but it can be a nuisance for beginning students. As you learn how to program in Java, there will be times when you will be asked to be satisfied with a preliminary explanation and wait for more complete detail in a later chapter. Java has been extended many times during its life—see Table 1. In this book, we assume that you have Java version 8 or later. Java has a very Finally, you cannot hope to learn all of Java in one course. The Java language itself large library. Focus is relatively simple, but Java contains a vast set of library packages that are required to on learning those write useful programs. There are packages for graphics, user-interface design, cryp- parts of the library that you need for tography, networking, sound, database storage, and many other purposes. Even your programming expert Java programmers cannot hope to know the contents of all of the packages— projects. they just use those that they need for particular projects. Using this book, you should expect to learn a good deal about the Java language and about the most important packages. Keep in mind that the central goal of this book is not to make you memorize Java minutiae, but to teach you how to think about programming. 1.4 Becoming Familiar with Your Programming Environment Set aside time to Many students find that the tools they need as programmers are very different from the become familiar with software with which they are familiar. You should spend some time making yourself the programming familiar with your programming environment. Because computer systems vary widely, environment that you will use for your this book can only give an outline of the steps you need to follow. It is a good idea to class work. participate in a hands-on lab, or to ask a knowledgeable friend to give you a tour. Step 1 Start the Java development environment. Computer systems differ greatly in this regard. On many computers there is an inte- grated development environment in which you can write and test your programs. bjeo_ch01p.indd 7 11/21/18 5:11 PM  8 Chapter 1 Introduction Figure 4 Running the HelloPrinter Program in an Integrated Click to compile and run Development Java program Environment Program output An editor is a On other computers you first launch an editor, a program that functions like a word program for entering processor, in which you can enter your Java instructions; you then open a console and modifying window and type commands to execute your program. You need to find out how to text, such as a Java program. get started with your environment. Step 2 Write a simple program. The traditional choice for the very first program in a new programming language is a program that displays a simple greeting: “Hello, World!”. Let us follow that tradi- tion. Here is the “Hello, World!” program in Java: public class HelloPrinter { public static void main(String[] args) { System.out.println("Hello, World!"); } } We will examine this program in the next section. No matter which programming environment you use, you begin your activity by typing the program statements into an editor window. Create a new file and call it HelloPrinter.java, using the steps that are appropriate for your environment. (If your environment requires that you supply a project name in addition to the file name, use the name hello for the project.) Enter the program instructions exactly as they are given above. Alternatively, locate the electronic copy in this book’s companion code and paste it into your editor. Java is case sensitive. As you write this program, pay careful attention to the various symbols, and keep You must be careful about distinguishing in mind that Java is case sensitive. You must enter upper- and lowercase letters exactly between upper- and as they appear in the program listing. You cannot type MAIN or PrintLn. If you are not lowercase letters. careful, you will run into problems—see Common Error 1.2. bjeo_ch01p.indd 8 11/21/18 5:11 PM  1.4 Becoming Familiar with Your Programming Environment 9 Figure 5 Running the HelloPrinter Program in a Console Window Step 3 Run the program. The process for running a program depends greatly on your programming environ- ment. You may have to click a button or enter some commands. When you run the test program, the message Hello, World! will appear somewhere on the screen (see Figure 4 and Figure 5). The Java compiler In order to run your program, the Java compiler translates your source files (that translates source is, the statements that you wrote) into class files. (A class file contains instructions for code into class the Java virtual machine.) After the compiler has translated your source code into files that contain instructions for the virtual machine instructions, the virtual machine executes them. During execution, Java virtual machine. the virtual machine accesses a library of pre-written code, including the implementa- tions of the System and PrintStream classes that are necessary for displaying the program’s output. Figure 6 summarizes the process of creating and running a Java program. In some programming environments, the compiler and virtual machine are essentially invisible to the programmer—they are automatically executed whenever you ask to run a Java program. In other environments, you need to launch the com- piler and virtual machine explicitly. Step 4 Organize your work. As a programmer, you write programs, try them out, and improve them. You store your programs in files. Files are stored in folders or directories. A folder can contain Virtual Editor Compiler Machine Class files Running Source File Program Library files Figure 6 From Source Code to Running Program bjeo_ch01p.indd 9 11/21/18 5:11 PM  10 Chapter 1 Introduction files as well as other folders, which themselves can contain more files and folders (see Figure 7). This hierarchy can be quite large, and you need not be concerned with all of its branches. However, you should create folders for organizing your work. It is a good idea to make a separate folder for your pro- gramming coursework. Inside that folder, make a separate folder for each program. Some programming environments place your programs into a default location if you don’t spec- ify a folder yourself. In that case, you need to find out where those files are located. Be sure that you understand where your files are located in the folder hierarchy. This information is essential when you submit files for grading, and for making backup copies (see Programming Tip 1.1). Figure 7 A Folder Hierarchy Programming Tip 1.1 Backup Copies You will spend many hours creating and improving Java programs. © Eric Isselé/iStockphoto. It is easy to delete a file by accident, and occasionally files are lost because of a computer malfunction. Retyping the contents of lost Develop a strategy files is frustrating and time-consuming. It is therefore crucially for keeping backup important that you learn how to safeguard files and get in the habit copies of your work of doing so before disaster strikes. Backing up files on a memory © Tatiana Popova/iStockphoto. before disaster strikes. stick is an easy and convenient storage method for many people. Another increasingly popular form of backup is Internet file storage. Here are a few pointers to keep in mind: Back up often. Backing up a file takes only a few seconds, and you will hate yourself if you have to spend many hours recreating work that you could have saved easily. I recommend that you back up your work once every thirty minutes. Rotate backups. Use more than one directory for backups, and rotate them. That is, first back up onto the first directory. Then back up onto the second directory. Then use the third, and then go back to the first. That way you always have three recent backups. If your recent changes made matters worse, you can then go back to the older version. Pay attention to the backup direction. Backing up involves copying files from one place to another. It is important that you do this right—that is, copy from your work location to the backup location. If you do it the wrong way, you will overwrite a newer file with an older version. Check your backups once in a while. Double-check that your backups are where you think they are. There is nothing more frustrating than to find out that the backups are not there when you need them. Relax, then restore. When you lose a file and need to restore it from a backup, you are likely to be in an unhappy, nervous state. Take a deep breath and think through the recovery process before you start. It is not uncommon for an agitated computer user to wipe out the last backup when trying to restore a damaged file. bjeo_ch01p.indd 10 11/21/18 5:11 PM  1.5 Analyzing Your First Program 11 1.5 Analyzing Your First Program In this section, we will analyze the first Java program in detail. Here again is the source code. sec04/HelloPrinter.java © Amanda Rohde/iStockphoto. 1 public class HelloPrinter 2 { 3 public static void main(String[] args) 4 { 5 // Display a greeting in the console window 6 7 System.out.println("Hello, World!"); 8 } © Amanda Rohde/iStockphoto. 9 } The line public class HelloPrinter indicates the declaration of a class called HelloPrinter. Classes are the Every Java program consists of one or more classes. We will discuss classes in more fundamental detail in Chapters 2 and 3. building blocks of The word public denotes that the class is usable by the “public”. You will later Java programs. encounter private features. In Java, every source file can contain at most one public class, and the name of the public class must match the name of the file containing the class. For example, the class HelloPrinter must be contained in a file named HelloPrinter.java. The construction public static void main(String[] args) {... } declares a method called main. A method contains a collection of programming instructions that describe how to carry out a particular task. Every Java Every Java application must have a main method. Most Java programs contain application contains other methods besides main, and you will see in Chapter 3 how to write other a class with a main methods. method. When the application starts, The term static is explained in more detail in Chapter 8, and the meaning of the instructions in String[] args is covered in Chapter 11. At this time, simply consider the main method are executed. public class ClassName { public static void main(String[] args) {... } } Each class contains as a part of the “plumbing” that is required to create a Java program. Our first pro- declarations of gram has all instructions inside the main method of the class. methods. Each method contains The main method contains one or more instructions called statements. Each state- a sequence ment ends in a semicolon (;). When a program runs, the statements in the main method of instructions. are executed one by one. bjeo_ch01p.indd 11 11/21/18 5:11 PM  12 Chapter 1 Introduction In our example program, the main method has a single statement: System.out.println("Hello, World!"); This statement prints a line of text, namely “Hello, World!”. In this statement, we call a method which, for reasons that we will not explain here, is specified by the rather long name System.out.println. We do not have to implement this method—the programmers who wrote the Java library already did that for us. We simply want the method to perform its intended task, namely to print a value. A method is called Whenever you call a method in Java, you need to specify by specifying the method and 1. The method you want to use (in this case, System.out.println). its arguments. 2. Any values the method needs to carry out its task (in this case, "Hello, World!"). The technical term for such a value is an argument. Arguments are enclosed in parentheses. Multiple arguments are separated by commas. A sequence of characters enclosed in quotation marks "Hello, World!" A string is a sequence is called a string. You must enclose the contents of the string inside quotation marks of characters so that the compiler knows you literally mean "Hello, World!". There is a reason for enclosed in this requirement. Suppose you need to print the word main. By enclosing it in quota- quotation marks. tion marks, "main", the compiler knows you mean the sequence of characters m a i n, not the method named main. The rule is simply that you must enclose all text strings in quotation marks, so that the compiler considers them plain text and does not try to interpret them as program instructions. You can also print numerical values. For example, the statement System.out.println(3 + 4); evaluates the expression 3 + 4 and displays the number 7. Syntax 1.1 Java Program Every program contains at least one class. Choose a class name that describes the program action. Every Java program public class HelloPrinter contains a main method { with this header. public static void main(String[] args) { System.out.println("Hello, World!"); Replace this The statements inside the } statement when you main method are executed } write your own when the program runs. Each statement programs. ends in a semicolon. See Common Error 1.1. Be sure to match the opening and closing braces. bjeo_ch01p.indd 12 11/21/18 5:11 PM  1.6 Errors 13 The System.out.println method prints a string or a number and then starts a new line. For example, the sequence of statements System.out.println("Hello"); System.out.println("World!"); prints two lines of text: Hello World! There is a second method, System.out.print, that you can use to print an item without starting a new line. For example, the output of the two statements System.out.print("00"); System.out.println(3 + 4); is the single line 007 EXAMPLE CODE See sec05 of your eText or companion code for a program that demonstrates print commands. Common Error 1.1 Omitting Semicolons © John Bell/iStockphoto. In Java every statement must end in a semicolon. Forgetting to type a semicolon is a common error. It confuses the compiler, because the compiler uses the semicolon to find where one statement ends and the next one starts. The compiler does not use line breaks or closing braces to recognize the end of statements. For example, the compiler considers System.out.println("Hello") System.out.println("World!"); a single statement, as if you had written System.out.println("Hello") System.out.println("World!"); Then it doesn’t understand that statement, because it does not expect the word System follow- ing the closing parenthesis after "Hello". The remedy is simple. Scan every statement for a terminating semicolon, just as you would check that every English sentence ends in a period. However, do not add a semicolon at the end of public class Hello or public static void main. These lines are not statements. 1.6 Errors Experiment a little with the HelloPrinter program. What happens if you make a typing error such as System.ou.println("Hello, World!"); System.out.println("Hello, Word!"); In the first case, the compiler will complain. It will say that it has no clue what you mean by ou. The exact wording of the error message is dependent on your develop- ment environment, but it might be something like “Cannot find symbol ou”. This is a compile-time error. Something is wrong according to the rules of the language and the compiler finds it. For this reason, compile-time errors are often called syntax errors. When the compiler finds one or more errors, it refuses to translate the bjeo_ch01p.indd 13 11/21/18 5:11 PM  14 Chapter 1 Introduction program into Java virtual machine instructions, and as a consequence you have no program that you can run. You must fix the error and compile again. In fact, the com- piler is quite picky, and it is common to go through several rounds of fixing compile- time errors before compilation succeeds for the first time. A compile-time error If the compiler finds an error, it will not simply is a violation of stop and give up. It will try to report as many the programming errors as it can find, so you can fix them all at once. language rules that is detected by Sometimes, an error throws the compiler the compiler. off track. Suppose, for example, you forget the quotation marks around a string: System.out. println(Hello, World!). The compiler will not complain about the missing quotation marks. Instead, it will report “Cannot find symbol Hello”. Unfortunately, the compiler is not very smart and it does not realize that you meant to © Martin Carlsson/iStockphoto. use a string. It is up to you to realize that you Programmers spend a fair amount need to enclose strings in quotation marks. of time fixing compile-time and The error in the second line above is of a dif- run-time errors. ferent kind. The program will compile and run, but its output will be wrong. It will print Hello, Word! A run-time error This is a run-time error. The program is syntactically correct and does something, causes a program to but it doesn’t do what it is supposed to do. Because run-time errors are caused by take an action that logical flaws in the program, they are often called logic errors. the programmer did not intend. This particular run-time error did not include an error message. It simply pro- duced the wrong output. Some kinds of run-time errors are so severe that they gen- erate an exception: an error message from the Java virtual machine. For example, if your program includes the statement System.out.println(1 / 0); you will get a run-time error message “Division by zero”. During program development, errors are unavoidable. Once a program is longer than a few lines, it would require superhuman concentration to enter it correctly without slipping up once. You will find yourself omitting semicolons or quotation marks more often than you would like, but the compiler will track down these prob- lems for you. Run-time errors are more troublesome. The compiler will not find them—in fact, the compiler will cheerfully translate any program as long as its syntax is correct— but the resulting program will do something wrong. It is the responsibility of the program author to test the program and find any run-time errors. EXAMPLE CODE See sec06 of your eText or companion code for three programs that illustrate errors. Common Error 1.2 Misspelling Words © John Bell/iStockphoto. If you accidentally misspell a word, then strange things may happen, and it may not always be completely obvious from the error messages what went wrong. Here is a good example of how simple spelling errors can cause trouble: bjeo_ch01p.indd 14 11/21/18 5:11 PM  1.7 Problem Solving: Algorithm Design 15 public class HelloPrinter { public static void Main(String[] args) { System.out.println("Hello, World!"); } } This class declares a method called Main. The compiler will not consider this to be the same as the main method, because Main starts with an uppercase letter and the Java language is case sen- sitive. Upper- and lowercase letters are considered to be completely different from each other, and to the compiler Main is no better match for main than rain. The compiler will cheerfully compile your Main method, but when the Java virtual machine reads the compiled file, it will complain about the missing main method and refuse to run the program. Of course, the mes- sage “missing main method” should give you a clue where to look for the error. If you get an error message that seems to indicate that the compiler or virtual machine is on the wrong track, check for spelling and capitalization. If you misspell the name of a symbol (for example, ou instead of out), the compiler will produce a message such as “cannot find sym- bol ou”. That error message is usually a good clue that you made a spelling error. 1.7 Problem Solving: Algorithm Design You will soon learn how to program calculations and decision making in Java. But before we look at the mechanics of implementing computations in the next chapter, let’s consider how you can describe the steps that are necessary for finding the solu- tion to a problem. 1.7.1 The Algorithm Concept You may have run across advertisements that encourage you to pay for a computerized service that matches you up with a love partner. Think how this might work. You fill out a form and send it in. Others do the same. The data are pro- cessed by a computer program. Is it reasonable to assume that the computer can perform the task of finding the best match for you? Suppose your younger brother, not the computer, had all the forms on his desk. What instructions could you give him? You can’t say, “Find the best-looking © mammamaart/iStockphoto. person who likes inline skating and browsing the Internet”. There is no objective standard for Finding the perfect partner is not a problem that a computer can solve. good looks, and your brother’s opinion (or that of a computer program analyzing the photos of prospective partners) will likely be different from yours. If you can’t give written instructions for someone to solve the problem, there is no way the computer can magically find the right solution. The computer can only do what you tell it to do. It just does it faster, without getting bored or exhausted. bjeo_ch01p.indd 15 11/21/18 5:11 PM  16 Chapter 1 Introduction For that reason, a computerized match-making service cannot guarantee to find the optimal match for you. Instead, you may be presented with a set of potential part- ners who share common interests with you. That is a task that a computer program can solve. In order for a computer program to provide an answer to a problem that computes an answer, it must follow a sequence of steps that is Unambiguous Executable Terminating An algorithm for The step sequence is unambiguous when there are solving a problem is precise instructions for what to do at each step and a sequence of steps where to go next. There is no room for guesswork that is unambiguous, executable, and or personal opinion. A step is executable when it terminating. can be carried out in practice. For example, a com- puter can list all people that share your hobbies, but it can’t predict who will be your life-long part- ner. Finally, a sequence of steps is terminating if it will eventually come to an end. A program that keeps working without delivering an answer is clearly not useful. A sequence of steps that is unambiguous, exe- cutable, and terminating is called an algorithm. © Claudiad/iStockphoto. Although there is no algorithm for finding a part- ner, many problems do have algorithms for solving An algorithm is a recipe for finding them. The next section gives an example. a solution. 1.7.2 An Algorithm for Solving an Investment Problem Consider the following investment problem: You put $10,000 into a bank account that earns 5 percent interest per year. How many years does it take for the account balance to be double the original? Could you solve this problem by hand? Sure, you could. You figure out the balance as follows: year interest balance 0 10000 1 10000.00 x 0.05 = 500.00 10000.00 + 500.00 = 10500.00 2 10500.00 x 0.05 = 525.00 10500.00 + 525.00 = 11025.00 3 11025.00 x 0.05 = 551.25 11025.00 + 551.25 = 11576.25 4 11576.25 x 0.05 = 578.81 11576.25 + 578.81 = 12155.06 You keep going until the balance is at least $20,000. Then the last number in the year column is the answer. Of course, carrying out this computation is intensely boring to you or your younger brother. But computers are very good at carrying out repetitive calcula- tions quickly and flawlessly. What is important to the computer is a description of the bjeo_ch01p.indd 16 11/21/18 5:11 PM  1.7 Problem Solving: Algorithm Design 17 steps for finding the solution. Each step must be clear and unambiguous, requiring no guesswork. Here is such a description: Set year to 0, balance to 10000. year interest balance 0 10000 While the balance is less than $20,000 Add 1 to the year. Set the interest to balance x 0.05 (i.e., 5 percent interest). Add the interest to the balance. year interest balance 0 10000 1 500.00 10500.00 14 942.82 19799.32 15 989.96 20789.28 Report year as the answer. These steps are not yet in a language that a computer can understand, but you will soon learn how to formulate them in Java. This informal description is called pseudo- code. We examine the rules for writing pseudocode in the next section. 1.7.3 Pseudocode Pseudocode is an There are no strict requirements for pseudocode because it is read by human readers, informal description not a computer program. Here are the kinds of pseudocode statements and how we of a sequence of will use them in this book: steps for solving a problem. Use statements such as the following to describe how a value is set or changed: total cost = purchase price + operating cost Multiply the balance value by 1.05. Remove the first and last character from the word. Describe decisions and repetitions as follows: If total cost 1 < total cost 2 While the balance is less than $20,000 For each picture in the sequence Use indentation to indicate which statements should be selected or repeated: For each car operating cost = 10 x annual fuel cost total cost = purchase price + operating cost Here, the indentation indicates that both statements should be executed for each car. bjeo_ch01p.indd 17 11/29/18 3:53 PM  18 Chapter 1 Introduction Indicate results with statements such as: Choose car1. Report year as the answer. 1.7.4 From Algorithms to Programs In Section 1.7.2, we developed pseudocode for finding how long it takes to double an Understand investment. Let’s double-check that the pseudocode represents an algorithm; that is, the problem that it is unambiguous, executable, and terminating. Our pseudocode is unambiguous. It simply tells how to update values in each step. The pseudocode is executable because we use a fixed interest rate. Had we said to use Develop and the actual interest rate that will be charged in years to come, and not a fixed rate of 5 describe an percent per year, the instructions would not have been executable. There is no way algorithm for anyone to know what the interest rate will be in the future. It requires a bit of thought to see that the steps are terminating: With every step, the balance goes up by at least $500, so eventually it must reach $20,000. Test the Therefore, we have found an algorithm to solve our investment problem, and algorithm with we know we can find the solution by programming a computer. The existence of simple inputs an algorithm is an essential prerequisite for programming a task. You need to first discover and describe an algorithm for the task before you start programming (see Figure 8). In the chapters that follow, you will learn how to express algorithms in the Translate Java language. the algorithm into Java Compile and test your program Figure 8 The Software Development Process HOW TO 1.1 Describing an Algorithm with Pseudocode This is the first of many “How To” sections in this book that give you step-by-step proce- dures for carrying out important tasks in developing computer programs. Before you are ready to write a program in Java, you need to develop an algorithm—a method for arriving at a solution for a particular problem. Describe the algorithm in pseudo- © Steve Simzer/iStockphoto. code—a sequence of precise steps formulated in English. To illustrate, we’ll devise an algo- rithm for this problem: Problem Statement You have the choice of buying one of two cars. One is more fuel efficient than the other, but also more expensive. You know the price and fuel efficiency (in miles per gallon, mpg) of both cars. You plan to keep the car for ten years. Assume a price of $4 per gallon of gas and usage of 15,000 miles per year. You will pay cash for the car and not worry about financing costs. Which car is the better deal? © David H. Lewis/Getty Images. bjeo_ch01p.indd 18 11/29/18 3:53 PM  1.7 Problem Solving: Algorithm Design 19 Step 1 Determine the inputs and outputs. In our sample problem, we have these inputs: purchase price1 and fuel efficiency1, the price and fuel efficiency (in mpg) of the first car purchase price2 and fuel efficiency2, the price and fuel efficiency of the second car We simply want to know which car is the better buy. That is the desired output. Step 2 Break down the problem into smaller tasks. For each car, we need to know the total cost of driving it. Let’s do this computation separately for each car. Once we have the total cost for each car, we can decide which car is the better deal. The total cost for each car is purchase price + operating cost. We assume a constant usage and gas price for ten years, so the operating cost depends on the cost of driving the car for one year. The operating cost is 10 x annual fuel cost. The annual fuel cost is price per gallon x annual fuel consumed. The annual fuel consumed is annual miles driven / fuel efficiency. For example, if you drive the car for 15,000 miles and the fuel efficiency is 15 miles/gallon, the car consumes 1,000 gallons. Step 3 Describe each subtask in pseudocode. In your description, arrange the steps so that any intermediate values are computed before they are needed in other computations. For example, list the step total cost = purchase price + operating cost after you have computed operating cost. Here is the algorithm for deciding which car to buy: For each car, compute the total cost as follows: annual fuel consumed = annual miles driven / fuel efficiency annual fuel cost = price per gallon x annual fuel consumed operating cost = 10 x annual fuel cost total cost = purchase price + operating cost If total cost of car1 < total cost of car2 Choose car1. Else Choose car2. Step 4 Test your pseudocode by working a problem. We will use these sample values: Car 1: $25,000, 50 miles/gallon Car 2: $20,000, 30 miles/gallon Here is the calculation for the cost of the first car: annual fuel consumed = annual miles driven / fuel efficiency = 15000 / 50 = 300 annual fuel cost = price per gallon x annual fuel consumed = 4 x 300 = 1200 operating cost = 10 x annual fuel cost = 10 x 1200 = 12000 total cost = purchase price + operating cost = 25000 + 12000 = 37000 Similarly, the total cost for the second car is $40,000. Therefore, the output of the algorithm is to choose car 1. The following Worked Example demonstrates how to use the concepts in this chap- ter and the steps in the How To to solve another problem. In this case, you will see bjeo_ch01p.indd 19 11/21/18 5:11 PM