You didn't think I'd actually provide the flat-out answer... did you? That's just not my MO :-D Seriously, do you own a Java book? Either way, I always recommend Head First Java, for starters - but there are many other choices. If you'd rather not read a book, I've taken the liberty of
linking to the specific topic in the Sun Java Tutorials, your answer is there. You must read and research this topic as its so fundamental and critical in Java. Let us know what you come up with.
