May I suggest you try searching the forum before asking questions such as this as a lot of these types of questions have already been answered and you will get your answer much quicker than waiting for one of us to reply to your post.
If you do read an earlier thread and don't understand the answer then by all means post a new thread saying something like I read thread "ABC" and still don't understand "XYZ".
Terence hiu wrote:I keep coming across the word abstract data type, what is it exactly? and if some tells me to design an ADT for a say CD collection, what is it do i do?
It also helps if you use standard English when posting. We can make allowances if it is not your first language, but I, for one, have know idea what an ADT is - although from your post I assume it's an acronym for your subject.
Just to add to Tony's good advice, and since "collection" seems to be the operative word, you might want to look at the docs for the Java Collections Framework, as it contains many interfaces and abstract classes that you may find useful.
Isn't it funny how there's always time and money enough to do it WRONG?