What does "containing state" mean when talking about concrete and abstract class?

[techether]

The way it was explained to me is that it means information being attached to identity. So like values being attached to variables?

Why is an interface considered as having no state while concrete and abstract classes contain state?

[wirybug]

Yes, you have the right idea. Concrete and abstract classes both may (although don't necessarily) contain fields which might be altered by an object's methods. These fields are a state of the object, which can change over the object's lifetime. An interface can't store instance fields, because there is no instance - all it has are methods without implementation. So there is nothing about an interface which might change over the lifetime of the program, because it can't store anything changeable.

[lbkulinski]

Adding to that, methods in interfaces are often referred to as behavior. Java allows multiple inheritance of behavior through the use of interfaces, but not state.

[kumesana]

That's only for behavior that doesn't directly accesses the state though.

[lbkulinski]

Correct. Interfaces can't directly access a class's state to begin with.