Sample Java Program

Sample Java Program - Source Code

Our standard sample count-to-ten program in Java:

/*


  Sample Java Program - Counts to Ten
 
  Copyright 2006 Andrew Eichstaedt
  Eichstaedt Development Group
  http://www.andrew-eichstaedt.com


*/


class CountToTen {

    public static void main(String args[]) {

        System.out.println("Hello!");
        System.out.println("I am a sample Java program");
        System.out.println("that counts to ten.");
        System.out.println("");

        for (int i = 1; i <= 10; i++) {      
            System.out.println(i);
        }
        
        System.out.println("");
        System.out.println("Thanks for running me.");
    }
}

General Notes on Java

Java is an object-oriented language with a syntax similar to C++. A major design objective of the Java language is cross-platform compatibility. Unlike languages like C or C++, which are compiled directly into native machine code, Java is typically compiled into into an intermediate byte code format, which is run by a Java Virtual Machine (JVM) on the particular architecture being used.

Java applications make use of the Java Runtime Environment (JRE). Installed on the computer running the application, the JRE includes the JVM and an extensive collection of class libraries.

Java applications may be run as command line programs, as in our example program above. They also may be run as applets, embedded into a web page; as servlets, running as processes on a server; or using a graphical user interface implemented with the Swing or Abstract Window Toolkit (AWT) libraries.

History of Java

Java began at Sun Microsystems in 1990 as part of a project to develop smart appliances. It was intended as a smaller-footprint replacement for C++, easily ported to a variety of devices. Another feature was the addition of automatic garbage collection, which prevents the programmer from having to explicitly handle memory management. By 1992, Java, originally named Oak, had appeared.

In 1994, the language was rechristened Java and re-targeted towards Web applications. The first public release of the Java Development Kit (JDK) version 1.0, became available in January of 1996. Several revisions of the language and the Java class library have been released since then, with more planned in upcoming years.

Using the Example Java Program

First we compile the program, contained in our source file sample-java-program.java, by invoking the Java compiler:

$ javac sample-java-program.java

This creates a bytecode class file named CountToTen.class. To run the program, we launch the Java Virtual Machine:

$ java CountToTen

to produce the output:

Hello! I am a sample Java program
that counts to ten.

1
2
3
4
5
6
7
8
9
10

Thanks for running me.

Sample Java Program - Comments for Programmers

The first portion of the code within /* ... */ is a normal C-style comment. C++-style comments, begun by two slashes at the end of the line, are also supported.

In Java, all working code exists within a class. In this case, we name our new class CountToTen. The class definition and other blocks of code are listed within curly braces ( { and } ) as in C or C++.

Some explanation is required for the first line within the class definition:

public static void main(String args[])

This line defines a method named main, which is special. The main method is the starting point for the JVM to begin execution of the program. A more complex problem than our sample will have many classes, but only one class needs a main method. Note that when writing Java applets, a main method is not used.

The public access specifier indicates that the method will be visible to any class. Other access specifiers are private and protected, used to detote other levels of visibility.

The static declaration indicates that main is a class method. This allows it to be run without instantiating a particular instance of the CountToTen class, which is necessary for it to be called by the JVM before any instances of the class are created.

The void declaration, as in C, indicates that no values is returned by the method.

The String args[] portion specifies that the method takes an array of zero or more strings as command-line arguments. In this case, we are using no command-line arguments, but the parameter is required for the main method.

The next block of lines is of the form System.out.println("Hello!"); simply prints out the the strings specified in double-quotes.

The counterintive sytntax of what would be a print statement in many langages exposes some of the complexity of Java's strict object model. The System class is a collection of useful objects and variables, including out, which is an output stream directed at standard output.

The for loop uses fairly standard C/C++-style syntax:

for (int i = 1; i <= 10; i++) {
    System.out.println(i);
}

It loops from one to ten, incrementing the variable i. The first parameter to the for statement, int i = 1, declares i as an integer and sets the value of i to one. The next parameter, i <= 10, is a test expression returning a boolean value. If value is true, the code in the block is run. The last parameter is also an expression, executed with each iteration of the loop. Here, i++ increments i by one, just as in C or C++.

The final block of println statements simply print the goodbye message.

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Copyright 2000-2006 Andrew Eichstaedt
Eichstaedt Development Group
Reynoldsburg, Ohio