A First Simple Program

 

Now that the basic object-oriented underpinning of Java has been discussed, let’s look at some actual Java programs. Let’s start by compiling and running the short sample program shown here. As you will see, this involves a little more work than you might imagine.

/*

This is a simple Java program.

Call this file “Example.java”.

*/

class Example {

// Your program begins with a call to main().

public static void main(String args[]) {

System.out.println(“This is a simple Java program.”);

}

}

NOTE The descriptions that follow use the standard Java SE 6 Development Kit (JDK 6), which is available from Sun Microsystems. If you are using a different Java development environment, then you may need to follow a different procedure for compiling and executing Java programs

Entering the Program

For most computer languages, the name of the file that holds the source code to a program is immaterial. However, this is not the case with Java. The first thing that you must learn about Java is that the name you give to a source file is very important. For this example, the name of the source file should be Example.java. Let’s see why. In Java, a source file is officially called a compilation unit. It is a text file that contains one or more class definitions. The Java compiler requires that a source file use the .java filename extension. As you can see by looking at the program, the name of the class defined by the program is also Example. This is not a coincidence. In Java, all code must reside inside a class. By convention, the name of that class should match the name of the file that holds the program. You should also make sure that the capitalization of the filename matches the class name. The reason for this is that Java is case-sensitive. At this point, the convention that filenames correspond to class names may seem arbitrary. However, this convention makes it easier to maintain and organize your programs.

Compiling the Program

To compile the Example program, execute the compiler, javac, specifying the name of the

source file on the command line, as shown here:

C:\>javac Example.java

The javac compiler creates a file called Example.class that contains the bytecode version of the program. The Java bytecode is the intermediate representation of your program that contains instructions the Java Virtual Machine will execute. Thus, the output of javac is not code that can be directly executed. To actually run the program, you must use the Java application launcher, called java.

To do so, pass the class name Example as a command-line argument, as shown here:

C:\>java Example

When the program is run, the following output is displayed:

This is a simple Java program.

When Java source code is compiled, each individual class is put into its own output file named after the class and using the .class extension. This is why it is a good idea to give your Java source files the same name as the class they contain—the name of the source file will match the name of the .class file. When you execute java as just shown, you are actually specifying the name of the class that you want to execute. It will automatically search for a file by that name that has the .class extension. If it finds the file, it will execute the code contained in the specified class.

A Closer Look at the First Sample Program

Although Example.java is quite short, it includes several key features that are common to all Java programs. Let’s closely examine each part of the program.

The program begins with the following lines:

/*

This is a simple Java program.

Call this file “Example.java”.

*/

This is a comment. Like most other programming languages, Java lets you enter a remark into a program’s source file. The contents of a comment are ignored by the compiler. Instead, a comment describes or explains the operation of the program to anyone who is reading its source code. In this case, the comment describes the program and reminds you that the source file should be called Example.java. Of course, in real applications, comments generally explain how some part of the program works or what a specific feature does.

Java supports three styles of comments. The one shown at the top of the program is called a multiline comment. This type of comment must begin with /* and end with */. Anything between these two comment symbols is ignored by the compiler. As the name suggests, a multiline comment may be several lines long. The next line of code in the program is shown here:

class Example {

This line uses the keyword class to declare that a new class is being defined. Example is an identifier that is the name of the class. The entire class definition, including all of its members, will be between the opening curly brace ({) and the closing curly brace (}). The next line in the program is the single-line comment, shown here:

// Your program begins with a call to main().

This is the second type of comment supported by Java. A single-line comment begins with a // and ends at the end of the line. As a general rule, programmers use multiline comments for longer remarks and single-line comments for brief, line-by-line descriptions. The third type of comment, a documentation comment.

The next line of code is shown here:

public static void main(String args[]) {

This line begins the main( ) method. As the comment preceding it suggests, this is the line at which the program will begin executing. All Java applications begin execution by calling main( ). The full meaning of each part of this line cannot be given now, since it involves a detailed understanding of Java’s approach to encapsulation. However, since most of the examples in the first part of this book will use this line of code, let’s take a brief look at each part now.

The public keyword is an access specifier, which allows the programmer to control the visibility of class members. When a class member is preceded by public, then that member may be accessed by code outside the class in which it is declared. (The opposite of public is private, which prevents a member from being used by code defined outside of its class.) In this case, main( ) must be declared as public, since it must be called by code outside of its class when the program is started. The keyword static allows main( ) to be called without having to instantiate a particular instance of the class. This is necessary since main( ) is called by the Java Virtual Machine before any objects are made. The keyword void simply tells the compiler that main( ) does not return a value. As you will see, methods may also return values.

As stated, main( ) is the method called when a Java application begins. Keep in mind that Java is case-sensitive. Thus, Main is different from main. It is important to understand that the Java compiler will compile classes that do not contain a main( ) method. But java has no way to run these classes. So, if you had typed Main instead of main, the compiler would still compile your program. However, java would report an error because it would be unable to find the main( ) method.

Any information that you need to pass to a method is received by variables specified within the set of parentheses that follow the name of the method. These variables are called parameters. If there are no parameters required for a given method, you still need to include the empty parentheses. In main( ), there is only one parameter, albeit a complicated one. String args[ ] declares a parameter named args, which is an array of instances of the class String.

(Arrays are collections of similar objects.) Objects of type String store character strings. In this case, args receives any command-line arguments present when the program is executed.

The last character on the line is the {. This signals the start of main( )’s body. All of the code that comprises a method will occur between the method’s opening curly brace and its closing curly brace.

One other point: main( ) is simply a starting place for your program. A complex program will have dozens of classes, only one of which will need to have a main( ) method to get things started. When you begin creating applets—Java programs that are embedded in web browsers—you won’t use main( ) at all, since the web browser uses a different means of starting the execution of applets. The next line of code is shown here. Notice that it occurs inside main( ).

System.out.println(“This is a simple Java program.”);

This line outputs the string “This is a simple Java program.” followed by a new line on the screen. Output is actually accomplished by the built-in println( ) method. In this case, println( ) displays the string which is passed to it. As you will see, println( ) can be used to display other types of information, too. The line begins with System.out. While too complicated to explain in detail at this time, briefly, System is a predefined class that provides access to the system, and out is the output stream that is connected to the console.

As you have probably guessed, console output (and input) is not used frequently in most real-world Java programs and applets. Since most modern computing environments are windowed and graphical in nature, console I/O is used mostly for simple utility programs and for demonstration programs. Notice that the println( ) statement ends with a semicolon. All statements in Java end with a semicolon. The reason that the other lines in the program do not end in a semicolon is that they are not, technically, statements. The first } in the program ends main( ), and the last } ends the Example class definition.

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