How does Java Object casting work behind the scene? [duplicate]

Answer 1

Accodring to this: checkcast, what it does is to verify if the reference is assignable. If it is, the stack is not changed and the operation on that reference are kept.

So if you have:

 Child c = ( Child )  anyObject; 
 c.sayHi();

If the cast success, then the method sayHi could be invoked:

If objectref can be cast to the resolved class, array, or interface type, the operand stack is unchanged; otherwise, the checkcast instruction throws a ClassCastException.

Here's the "bytecode"

$ cat CastDemo.java 
class Parent {}
class Child extends Parent {}
class Main {
    Child c = (Child) new Parent();
}
$ javap -c Main
Compiled from "CastDemo.java"
class Main {
  Child c;

  Main();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."":()V
       4: aload_0       
       5: new           #2                  // class Parent
       8: dup           
       9: invokespecial #3                  // Method Parent."":()V
      12: checkcast     #4                  // class Child
      15: putfield      #5                  // Field c:LChild;
      18: return        
}