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A very simple Java application looks like this:

public class HelloWorld 
{
    public static void main(String[] args)
    {
        System.out.println("Hello World!");
    }
}

Program Structure

A class has:

• a name (here, HelloWorld)
• some number of methods
• some number of class variables

• a method called main with the following signature:

	public static void main(String[] args)

This means that it:

• has a single argument of type array of String
• returns void -- i.e., nothing

If that class doesn't have a main method with the proper signature -- Error!

Objects, Objects

In the above HelloWorld example, the main method's body calls the println method of the System.out object.

Command Line Arguments

Unlike C and C++, the command line arguments do not include the program name

public class EchoArguments
{
    public static void main(String[] args)
    {
        System.out.print(args.length);
	System.out.println(" arguments passed:");
        for (int arg = 0; arg < args.length; arg++)
            System.out.print("'" + args[arg] + "' ");
        System.out.println("");
    }
}

If the above program is run as follows:

java [options] EchoArguments fee "fi fo" fum 12 Englishmen at 0.5 apiece

will output:

8 arguments passed:
'fee' 'fi fo' 'fum' '12' 'Englishmen' 'at' '0.5' 'apiece'

Note that, normally, command line arguments are separated by whitespace.  However, you can enclose a single argument within quotes to override this behavior.

Program Exit Value

If you wish to return a numeric value to the system, as a normal C or C++ program would, you must call the System.exit method:

public static void main(String[] args)
{
    System.out.println("Hello World!");
    System.exit(0);
}

Environment:

However, Java does support a platform-independent system properties list:

public class PropertiesDisplayer
{
  public static void main(String[] args)
  {
    String home = System.getProperty("user.home");
    System.out.println("HOME = " + home);
    String classPath = System.getProperty("java.class.path");
    System.out.println("CLASSPATH = " + classPath);
    System.getProperties().list(System.out);
  }
}

When I ran the above program on my system (from within NetBeans), it gave me the following output:

HOME = C:\Documents and Settings\Bryan Higgs
CLASSPATH = C:\Documents and Settings\Bryan Higgs\My Documents\Java Projects\Test\build\classes
-- listing properties --
java.runtime.name=Java(TM) 2 Runtime Environment, Stand...
sun.boot.library.path=C:\Program Files\Java\jdk1.5.0\jre\bin
java.vm.version=1.5.0-b64
java.vm.vendor=Sun Microsystems Inc.
java.vendor.url=http://java.sun.com/
path.separator=;
java.vm.name=Java HotSpot(TM) Client VM
file.encoding.pkg=sun.io
user.country=US
sun.os.patch.level=Service Pack 2
java.vm.specification.name=Java Virtual Machine Specification
user.dir=C:\Documents and Settings\Bryan Higgs...
java.runtime.version=1.5.0-b64
java.awt.graphicsenv=sun.awt.Win32GraphicsEnvironment
java.endorsed.dirs=C:\Program Files\Java\jdk1.5.0\jre\li...
os.arch=x86
java.io.tmpdir=C:\DOCUME~1\BRYANH~1\LOCALS~1\Temp\
line.separator=

java.vm.specification.vendor=Sun Microsystems Inc.
user.variant=
os.name=Windows XP
sun.jnu.encoding=Cp1252
java.library.path=C:\Program Files\Java\jdk1.5.0\bin;.;...
java.specification.name=Java Platform API Specification
java.class.version=49.0
sun.management.compiler=HotSpot Client Compiler
os.version=5.1
user.home=C:\Documents and Settings\Bryan Higgs
user.timezone=
java.security.policy=applet.policy
java.awt.printerjob=sun.awt.windows.WPrinterJob
file.encoding=Cp1252
java.specification.version=1.5
user.name=Bryan Higgs
java.class.path=C:\Documents and Settings\Bryan Higgs...
java.vm.specification.version=1.0
sun.arch.data.model=32
java.home=C:\Program Files\Java\jdk1.5.0\jre
java.specification.vendor=Sun Microsystems Inc.
user.language=en
awt.toolkit=sun.awt.windows.WToolkit
java.vm.info=mixed mode, sharing
java.version=1.5.0
java.ext.dirs=C:\Program Files\Java\jdk1.5.0\jre\li...
sun.boot.class.path=C:\Program Files\Java\jdk1.5.0\jre\li...
java.vendor=Sun Microsystems Inc.
file.separator=\
java.vendor.url.bug=http://java.sun.com/cgi-bin/bugreport...
sun.cpu.endian=little
sun.io.unicode.encoding=UnicodeLittle
sun.desktop=windows
sun.cpu.isalist=pentium_pro+mmx pentium_pro pentium+m...

As you can see, even the default system properties provide quite a bit of information.

You can also pass name/value pairs into the Java program by using the -D option on the command line.

This page was last modified on 02 October, 2007