Possible Duplicates:
Why does Java allow multiple inheritance from interfaces but not from abstract/concrete classes
Why there is no multiple inheritance in Java, but implementing multiple interfaces is allowed
Instead of inheriting from multiple classes (which Java doesn’t allow), why are we told to implement multiple interfaces instead?
Surely the point of inheriting from multiple classes is the inherit their functionality – if you have to manually re-insert functionality (for each class extending a set of interfaces) what’s the point of using the interfaces? There’s no guarantee that two classes implementing the same set of interfaces will provide the same functionality – or am I missing something?
Multiple inheritance is something that can cause multiple problems.
Interfaces are used to give abilities to instances of the class that implement them. I personally use interfaces with composition(using instance variables that are references to other objects) in order to provide functionality to my class that would otherwise be achieved with multiple inheritance.
In other words my class provides the functionality promised by the interface implemented but internally my class instance uses the instance variable to do the job.
About your statement above:
Each method should adhere to a contract so no matter how you implement it the functionality of the method should always be the same if this is what is supposed to do. If it breaks the contract it means it was implemented wrongly.