Java Bytecode DUP

Question

I am wondering why the Exception in the following bytecode (used to throw an Exception) is duplicated.

NEW java/lang/IllegalArgumentException
DUP
INVOKESPECIAL j         


        

Answer 1

I'll analyze this line by line where [] = new stack after that op is used:

1. NEW puts a new IllegalArgumentException onto the stack [SomeIllegalArgumentException]
2. DUP duplicates it [SomeIllegalArgumentException, SomeIllegalArgumentException]
3. INVOKESPECIAL pops off the top one and initializes it by calling it's method [SomeIllegalArgumentException] (The init method will not return the object to put back onto the stack, so the object must first be duplicated so as to keep it on the stack)
4. ATHROW Throws the other (a duplicate off the one we initialized) []