Why use aload_0 in Jasmin HelloWorld?

Question

There is the start of official Jasmin HelloWorld example.

.class public NoJad.j
.super java/lang/Object

.method public <init>()V
   aload_0

   invokenonvirtual java/lang/Object/<init>()V
   return
.end method

.method public static main([Ljava/lang/String;)V
   .limit stack 2
...

Why there is aload_0 instruction? What does local var 0 consists?

Answer 1

It is calling a constructor of a base class (Object in this case):

this.super();

In non-static methods local variable under index 0 is always this reference so aload_0 pushes this reference at the top of the reference stack. Then invokenonvirtual calls designated method (Object/<init>()) on an object at the top of the stack (this).