HelloWorld program to a JVM code

Question

Let's consider the following piece of code:

package p;
public class Simple {

public static void main(String[] args){
    System.out.println("HELLO!\n");
   }
}

and JVM code for that ( javap -c Simple.class):

Compiled from "Simple.java"

public class p.Simple {
  public p.Simple();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #3                  // String HELLO!\n
       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: return
}

And I have a questions:

1. Probably, the entry point is the main function. But, before that, it is neccesary to invoke a constructor for Simple class. So, how does it work? I mean, who calls firstly the construactor and then pass a control to main? If I am wrong please explain me how the control flow works.

2. Instructions like getstatic take arguments like #2 . What does it mean #2?

3. Comment looks like: // Method java/lang/Object."<init>":()V What does it mean V?

4. How does the control flow work?

Answer 1

1. The Java compiler automatically generates a constructor when none is defined explicitly. In your example, it's not called at all. It's just that autogeneration policy.
2. It is a reference to the PrintStream property out on System. The Java compiler automatically converts import statements into fully qualified object declarations.
3. V means void
4. Probably something like this:
   1. Load the reference.
   2. Create an anonymous String object with that value.
   3. Push a stack frame into place that jumps to the println method's entry point with reference to #2 as the parameter.

What #2 means:

import java.util.List;

List x;

becomes simply this:

java.util.List x = new java.util.ArrayList();

when the compiler generates bytecode. java.lang is automatically imported.

Answer 2

1. Constructor is not called in this case because main() is static.

2. getstatic gets the value of System.out static field

3. V means void, that is main() return type is void