A First Program: Hello World!

The simplest version of this program consists of a single statement:

print("Hello world!")

The PVM simply executes this statement.

Another version embeds this statement in a main function which is called at the end of the module:

def main():
    print("Hello world!")

main()

The PVM executes the function definition and then calls the function.

The simplest version of this program defines a main method within a class:

public class HelloWorld{

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

The source code defines but does not call this method.  At program startup, the compiled class HelloWorld is loaded into the JVM.  The JVM then calls main, which is a class method in HelloWorld.

A Java application must include at least one class that defines a main method.  The byte code file for this class is the entry point for program execution.