If you like housing metaphors a class is like the blueprint for a house. Using this blueprint, you can build as many houses as you like. each house you build (or instantiate, in OO lingo) is an object , also known as an instance . each house also has an address, of course. If you want to tell someone where the house is, you give them a card with the address written on it. That card is the object's reference . If you want to visit the house, you look at the address written on the card. This is called dereferencing . You can copy that reference as much as you like, but there's just one house -- you're just copying the card that has the address on it, not the house itself. Java methods are always pass-by-value, but the value could be an object's reference. So, if I have: Foo myFoo = new Foo (); // 1 callBar ( myFoo ); // 2 myFoo . doSomething () // 4 void callBar ( Foo foo ) { foo = new Foo (); ...
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