OBJECT ORIENTED PROGRAMMING THROUGH JAVA : Unit - 1 : Program structure in Java

 Writing Simple Java Programs

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. The Java compiler requires that a source file use the .java file name 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 the main 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.

 

Example:

/*

This is a simple Java program. Call this file "Sample.java".

*/

class Sample{

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

 public static void main(String args[])

{

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

}

}

In Java, all code must reside inside a class. By convention, the name of the main 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.

The contents of a comment are ignored by the compiler. 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 Sample {

This line uses the keyword class to declare that a new class is being defined. Sample 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. The third type of comment, a documentation comment, will be discussed in the “Comments” section later in this chapter.

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.

The public keyword is an access modifier, 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 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.

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.

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.

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.

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 Sample class definition.

Compiling and Running Java Programs

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 Sample.class that contains the byte code version of the program. As discussed earlier, the Java byte code 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.