Hi, welcome to JavaRanch
I've edited your post to add code tags to make it more readable, please take the time to read about code tags in our FAQ entry here: UseCodeTags.
Now, as for your problem, you've defined a method called isEmpty(), but you're not actually calling it anywhere in your code eventhough you intended to do so.
Look carefully at lines 33, 54 and 73. Those aren't valid method calls.
Edit: ugh, beaten by Maneesh while making a coffee-run ;)
Build a man a fire, and he'll be warm for a day. Set a man on fire, and he'll be warm for the rest of his life.