Elements of Programming
Writing your first program is easier than writing a paragraph of text! Just a few building blocks suffice to enable us to write programs that can help solve all sorts of fascinating, but otherwise unapproachable, problems. This chapter takes you through these building blocks, gets you started on programming in Java, and studies a variety of interesting programs.
This chapter is from the book
This chapter is from the book
This chapter is from the book
OUR GOAL IN THIS CHAPTER IS to convince you that writing a program is easier than writing a piece of text, such as a paragraph or essay. Writing prose is difficult: we spend many years in school to learn how to do it. By contrast, just a few building blocks suffice to enable us to write programs that can help solve all sorts of fascinating, but otherwise unapproachable, problems. In this chapter, we take you through these building blocks, get you started on programming in Java, and study a variety of interesting programs. You will be able to express yourself (by writing programs) within just a few weeks. Like the ability to write prose, the ability to program is a lifetime skill that you can continually refine well into the future.
In this book, you will learn the Java programming language. This task will be much easier for you than, for example, learning a foreign language. Indeed, programming languages are characterized by only a few dozen vocabulary words and rules of grammar. Much of the material that we cover in this book could be expressed in the Python or C++ languages, or any of several other modern programming languages. We describe everything specifically in Java so that you can get started creating and running programs right away. On the one hand, we will focus on learning to program, as opposed to learning details about Java. On the other hand, part of the challenge of programming is knowing which details are relevant in a given situation. Java is widely used, so learning to program in this language will enable you to write programs on many computers (your own, for example). Also, learning to program in Java will make it easy for you to learn other languages, including lower-level languages such as C and specialized languages such as Matlab.
1.1 Your First Program
IN THIS SECTION, OUR PLAN IS to lead you into the world of Java programming by taking you through the basic steps required to get a simple program running. The Java platform (hereafter abbreviated Java) is a collection of applications, not unlike many of the other applications that you are accustomed to using (such as your word processor, email program, and web browser). As with any application, you need to be sure that Java is properly installed on your computer. It comes preloaded on many computers, or you can download it easily. You also need a text editor and a terminal application. Your first task is to find the instructions for installing such a Java programming environment on your computer by visiting
http://introcs.cs.princeton.edu/java
We refer to this site as the booksite. It contains an extensive amount of supplementary information about the material in this book for your reference and use while programming.
Programming in Java
To introduce you to developing Java programs, we break the process down into three steps. To program in Java, you need to:
Create a program by typing it into a file named, say, MyProgram.java.
Compile it by typing javac MyProgram.java in a terminal window.
Execute (or run) it by typing java MyProgram in the terminal window.
In the first step, you start with a blank screen and end with a sequence of typed characters on the screen, just as when you compose an email message or an essay. Programmers use the term code to refer to program text and the term coding to refer to the act of creating and editing the code. In the second step, you use a system application that compiles your program (translates it into a form more suitable for the computer) and puts the result in a file named MyProgram.class. In the third step, you transfer control of the computer from the system to your program (which returns control back to the system when finished). Many systems have several different ways to create, compile, and execute programs. We choose the sequence given here because it is the simplest to describe and use for small programs.
Creating a program
A Java program is nothing more than a sequence of characters, like a paragraph or a poem, stored in a file with a .java extension. To create one, therefore, you need simply define that sequence of characters, in the same way as you do for email or any other computer application. You can use any text editor for this task, or you can use one of the more sophisticated integrated development environments described on the booksite. Such environments are overkill for the sorts of programs we consider in this book, but they are not difficult to use, have many useful features, and are widely used by professionals.
Compiling a program
At first, it might seem that Java is designed to be best understood by the computer. To the contrary, the language is designed to be best understood by the programmer—that’s you. The computer’s language is far more primitive than Java. A compiler is an application that translates a program from the Java language to a language more suitable for execution on the computer. The compiler takes a file with a .java extension as input (your program) and produces a file with the same name but with a .class extension (the computer-language version). To use your Java compiler, type in a terminal window the javac command followed by the file name of the program you want to compile.
Executing (running) a program
Once you compile the program, you can execute (or run) it. This is the exciting part, where your program takes control of your computer (within the constraints of what Java allows). It is perhaps more accurate to say that your computer follows your instructions. It is even more accurate to say that a part of Java known as the Java virtual machine (JVM, for short) directs your computer to follow your instructions. To use the JVM to execute your program, type the java command followed by the program name in a terminal window.
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Program 1.1.1 Hello, World
public class HelloWorld
{
public static void main(String[] args)
{
// Prints "Hello, World" in the terminal window.
System.out.println("Hello, World");
}
}
This code is a Java program that accomplishes a simple task. It is traditionally a beginner’s first program. The box below shows what happens when you compile and execute the program. The terminal application gives a command prompt (% in this book) and executes the commands that you type (javac and then java in the example below). Our convention is to highlight in boldface the text that you type and display the results in regular face. In this case, the result is that the program prints the message Hello, World in the terminal window.
% javac HelloWorld.java % java HelloWorld Hello, World
PROGRAM 1.1.1 is an example of a complete Java program. Its name is HelloWorld, which means that its code resides in a file named HelloWorld.java (by convention in Java). The program’s sole action is to print a message to the terminal window. For continuity, we will use some standard Java terms to describe the program, but we will not define them until later in the book: PROGRAM 1.1.1 consists of a single class named HelloWorld that has a single method named main(). (When referring to a method in the text, we use () after the name to distinguish it from other kinds of names.) Until SECTION 2.1, all of our classes will have this same structure. For the time being, you can think of “class” as meaning “program.”
The first line of a method specifies its name and other information; the rest is a sequence of statements enclosed in curly braces, with each statement typically followed by a semicolon. For the time being, you can think of “programming” as meaning “specifying a class name and a sequence of statements for its main() method,” with the heart of the program consisting of the sequence of statements in the main() method (its body). PROGRAM 1.1.1 contains two such statements:
The first statement is a comment, which serves to document the program. In Java a single-line comment begins with two '/' characters and extends to the end of the line. In this book, we display comments in gray. Java ignores comments—they are present only for human readers of the program.
The second statement is a print statement. It calls the method named System.out.println() to print a text message—the one specified between the matching double quotes—to the terminal window.
In the next two sections, you will learn about many different kinds of statements that you can use to make programs. For the moment, we will use only comments and print statements, like the ones in HelloWorld.
When you type java followed by a class name in your terminal window, the system calls the main() method that you defined in that class, and executes its statements in order, one by one. Thus, typing java HelloWorld causes the system to call the main() method in PROGRAM 1.1.1 and execute its two statements. The first statement is a comment, which Java ignores. The second statement prints the specified message to the terminal window.
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Since the 1970s, it has been a tradition that a beginning programmer’s first program should print Hello, World. So, you should type the code in PROGRAM 1.1.1 into a file, compile it, and execute it. By doing so, you will be following in the footsteps of countless others who have learned how to program. Also, you will be checking that you have a usable editor and terminal application. At first, accomplishing the task of printing something out in a terminal window might not seem very interesting; upon reflection, however, you will see that one of the most basic functions that we need from a program is its ability to tell us what it is doing.
For the time being, all our program code will be just like PROGRAM 1.1.1, except with a different sequence of statements in main(). Thus, you do not need to start with a blank page to write a program. Instead, you can
Copy HelloWorld.java into a new file having a new program name of your choice, followed by .java.
Replace HelloWorld on the first line with the new program name.
Replace the comment and print statements with a different sequence of statements.
Your program is characterized by its sequence of statements and its name. Each Java program must reside in a file whose name matches the one after the word class on the first line, and it also must have a .java extension.
Errors
It is easy to blur the distinctions among editing, compiling, and executing programs. You should keep these processes separate in your mind when you are learning to program, to better understand the effects of the errors that inevitably arise.
You can fix or avoid most errors by carefully examining the program as you create it, the same way you fix spelling and grammatical errors when you compose an email message. Some errors, known as compile-time errors, are identified when you compile the program, because they prevent the compiler from doing the translation. Other errors, known as run-time errors, do not show up until you execute the program.
In general, errors in programs, also commonly known as bugs, are the bane of a programmer’s existence: the error messages can be confusing or misleading, and the source of the error can be very hard to find. One of the first skills that you will learn is to identify errors; you will also learn to be sufficiently careful when coding, to avoid making many of them in the first place. You can find several examples of errors in the Q&A at the end of this section.
Program 1.1.2 Using a command-line argument
public class UseArgument
{
public static void main(String[] args)
{
System.out.print("Hi, ");
System.out.print(args[0]);
System.out.println(". How are you?");
}
}
This program shows the way in which we can control the actions of our programs: by providing an argument on the command line. Doing so allows us to tailor the behavior of our programs.
% javac UseArgument.java % java UseArgument Alice Hi, Alice. How are you? % java UseArgument Bob Hi, Bob. How are you?
Input and output
Typically, we want to provide input to our programs—that is, data that they can process to produce a result. The simplest way to provide input data is illustrated in UseArgument (PROGRAM 1.1.2). Whenever you execute the program UseArgument, it accepts the command-line argument that you type after the program name and prints it back out to the terminal window as part of the message. The result of executing this program depends on what you type after the program name. By executing the program with different command-line arguments, you produce different printed results. We will discuss in more detail the mechanism that we use to pass command-line arguments to our programs later, in SECTION 2.1. For now it is sufficient to understand that args[0] is the first command-line argument that you type after the program name, args[1] is the second, and so forth. Thus, you can use args[0] within your program’s body to represent the first string that you type on the command line when it is executed, as in UseArgument.
In addition to the System.out.println() method, UseArgument calls the System.out.print() method. This method is just like System.out.println(), but prints just the specified string (and not a newline character).
Again, accomplishing the task of getting a program to print back out what we type in to it may not seem interesting at first, but upon reflection you will realize that another basic function of a program is its ability to respond to basic information from the user to control what the program does. The simple model that UseArgument represents will suffice to allow us to consider Java’s basic programming mechanism and to address all sorts of interesting computational problems.
Stepping back, we can see that UseArgument does neither more nor less than implement a function that maps a string of characters (the command-line argument) into another string of characters (the message printed back to the terminal window). When using it, we might think of our Java program as a black box that converts our input string to some output string.
A bird's-eye view of a Java program
This model is attractive because it is not only simple but also sufficiently general to allow completion, in principle, of any computational task. For example, the Java compiler itself is nothing more than a program that takes one string of characters as input (a .java file) and produces another string of characters as output (the corresponding .class file). Later, you will be able to write programs that accomplish a variety of interesting tasks (though we stop short of programs as complicated as a compiler). For the moment, we will live with various limitations on the size and type of the input and output to our programs; in SECTION 1.5, you will see how to incorporate more sophisticated mechanisms for program input and output. In particular, you will see that we can work with arbitrarily long input and output strings and other types of data such as sound and pictures.