Language Fundamentals - main()

Syntax

    public static void main(String[] args) {
        // method body
    }

• entry point for a Java application
• required by all Java applications (not required in Applets)
• must be declared public static void
• void must appear before main()

  Example:
      static public void main(String[] args){}    // legal
      public static void main(String[] args){}    // legal
      public void static main(String[] args){}    // illegal
  

• can also be declared final
• main() has only one argument: a String array
• the argument can be declared in many ways and the variable name does not have to be args

  Example:
  
          main( String args[] )
          main( String [] args )
          main( String[] params )
          main( String[] args )     // standard convention
  

• the args array is used to access command line arguments

  Example:
   java MyApp test this out 
  

• the args array uses a zero based index therefore args[0] would return "test" in the above example

• an application can have more than one main() method as every class can have a main() method
• which main() is used by an application depends on the class started at runtime
• advantage is that each class can use it's own main() as a testing structure for the class
• main() is inherited and can be overridden if not declared final

Code compiled with JDK 1.3 will work ok even it is declared private, protected or has no access modifier; however, for the purpose of the certification exam the correct method declaration is
public static void main(String[] varname)
(see discussion at JavaRanch)

Code Examples

• TestMain.java

Tips

• main() can be declared final
• main() is inherited and can be overridden if not declared as final
• args[0] references first command line argument after the application name ( arrays in Java are zero-based)
• main() can be declared public static void ... or static public void ...
• the variable name does not have to be args; can be anything as long as the type is String[]

Traps

• main() declared other than according to the standard convention