I think of objects as houses, and reference variables as rolodex cards
, or entires in your address book.
Anytime you see "new something", a house is built.
Anytime you see "variableName =", you are writing an address on the card.
GarbageCollection gc1 = new GarbageCollection();
GarbageCollection gc2 = new GarbageCollection();
builds a house, and writes the address of that house on gc1. Then it builds a second house, and writes that address on gc2.
puts a blank address card in your rolodex, and says "this is where I will write down the address for gc3".
This erases the old address on gc1, and replaces it with the address on gc2. Note that nothing about gc2 changes. You now have two variables with the same address. No card holds the address of the first thing you built. You have no way to find it - it is now eligible for garbage collection.
your doStuff() method does indeed return the address of a newly created object. You can call doStuff and ignore what it returns, which means you could create an object that is immediately eligible for gc, but this case, you do save it in your gc3 variable.