Why should the interface for a Java class be preferred?

Question

PMD would report a violation for:

ArrayList<Object> list = new ArrayList<Object>(); 

The violation was "Avoid using implementation types like 'ArrayList'; use the interface instead".

The following line would correct the violation:

List<Object> list = new ArrayList<Object>(); 

Why should the latter with List be used instead of ArrayList?

Answer 1

Using interfaces over concrete types is the key for good encapsulation and for loose coupling your code.

It's even a good idea to follow this practice when writing your own APIs. If you do, you'll find later that it's easier to add unit tests to your code (using Mocking techniques), and to change the underlying implementation if needed in the future.

Here's a good article on the subject.

Hope it helps!

Answer 2

This is preferred because you decouple your code from the implementation of the list. Using the interface lets you easily change the implementation, ArrayList in this case, to another list implementation without changing any of the rest of the code as long as it only uses methods defined in List.