java.util package - classes vs interfaces

Question

Why is Queue an interface, but others like Stack and ArrayList are classes?

I understand that interfaces are made so that clients can implement them and add on their own methods, whereas with classes if every client needs their methods in there it will become huge and bloated..

...or am I missing something here?

Answer 1

A Queue can be implemented in a number of fashions, as can a List or a Set. They all merely specify a contract for different kinds of collections.

An ArrayList, however, is a particular implementation of a List, made to internally use an array for storing elements. LinkedList is also an implementation of a List, which uses a series of interconnected nodes, i.e. a doubly linked list. Similarly, TreeSet and HashMap are particular implementations of sets and maps, respectively.

Now, Stack is a odd case here, particularly because it is a legacy class from older versions of Java. You really shouldn't use a Stack anymore; instead, you should use its modern equivalent, the ArrayDeque. ArrayDeque is an implementation of a Deque (a double-ended queue), that internally uses an array for storage (which is just what Stack does). A Deque supports all of the operations of a Stack, like pop, push, etc. Other implementations of Deque include LinkedList, as mentioned by someone else, although this deviates from Stack in that underlying it is not an array, but a doubly-linked list :-p

Now, there are plenty of implementations of Queue, and many different types of Queues. You not only have BlockingQueues (often used for producer-consumer), whose common implementations include LinkedBlockingQueue and ArrayBlockingQueue, but also TransferQueues, and so on. I digress... you can read more on the collections API in the relevant Java Tutorial.

Answer 2

You get the idea of interfaces correctly. In this case Java standard library already provides both implementations and interfaces. You are better of using an interface so you can switch the implementation any time.

Hope it makes sense.