What are the valid signatures of the main() function of a class?

The Java literature frequently refers to the Signature of a method. A method signature is a collection of information about the method, and that includes the name, type (e.g., static or non-static), visibility (e.g., public, private, etc.), arguments (e.g., formal parameters), and return type.

The main method is the entry point of the JVM when the class in launched. The JVM launchs the Java program by invoking the main method of the class identified in the command to start the program. The method main must be declared public, static, and void. It must accept a single argument that is an array of strings.

The main method can be declared as either:

public static void main(String[] args)

or

public static void main(String args[])

A compile-time error will occurs, if more than one main methods exist in your code with the signatures described in the above. For example, you will get compile-time error when you try to compile the following code.

public class Program{
  public static void main(String[] args) {
  }

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

You can have more than one methods with name "main" but have different other signatures. The JVM only looks for the main method described in the above at starting time. For example, the following code compiles and output "calling from public static void main(String[] args)".

public class Program{
  public static void main(String[] args) {
        main("calling from public static void main(String[] args)");
  }

  public static void main(String arg){
    System.out.println(arg);
  }
}

Is it allowed to declare the main method private?

Former JVM versions (pre-1.4) allowed the main method to have any accessibility (private, protected, etc). This incompatibility with the Section 12.1.4 of the Java Language Specification has been fixed as of version 1.4. In order to invoke a main method from the command-line, it is now mandatory to declare the main method as the above. If the main method has any access level other than public, it will no longer run from the command-line.

Printer-friendly version | Mail this to a friend