why do you create interfaces, and when MUST you use one?

As many people have probably already given their Idea, interfaces can be used to enforce certain behaviors between classes that will not implement those behaviors the same way. So by implementing an interface you are saying that your class has the behavior of the interface.

Interfaces are more flexible, because a class can implement multiple interfaces. Since Java does not have multiple inheritance, using abstract classes prevents your users from using any other class hierarchy. In general, prefer interfaces when there are no default implementations or state.