Java Interfaces Methodology: Should every class implement an interface?

Question

I've been programming in Java for a few courses in the University and I have the following question:

Is it methodologically accepted that every class should implement an interface? Is it considered bad practice not to do so? Can you describe a situation where it's not a good idea to use interfaces?

Edit: Personally, I like the notion of using Interfaces for everything as a methodology and habit, even if it's not clearly beneficial. Eclipse automatically created a class file with all the methods, so it doesn't waste any time anyway.

Answer 1

You don't need to create an interface if you are not going to use it.

Typically you need an interface when:

• Your program will provide several implementations for your component. For example, a default implementation which is part of your code, and a mock implementation which is used in a JUnit test. Some tools automate creating a mock implementation, like for instance EasyMock.
• You want to use dependency injection for this class, with a framework such as Spring or the JBoss Micro-Container. In this case it is a good idea to specify the dependencies from one class with other classes using an interface.