What you've shown is casting. Let's expand it a bit to make sure we're on the same page:
Because B extends A, somebody could create an instance of B and pass it to myMethod. Inside myMethod, I'm so confident that this really is an instance of B, I use casting to tell the compiler to trust me and let me make my obj variable refer to a1.
What if I'm wrong and somebody calls myMethod with an instance of A? I've convinced the compiler to let me try this, and it won't get caught until runtime. Then I'll get a class cast exception.
Now, since you said "copy" instead of "cast" I'm not sure this is even what you wanted to know. Is it close?
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi
In java, usually you have to call .clone() to make a real copy, what your code does is simply make another reference to the same object, not have two objects.
