what is the advantage of interface over abstract classes?

Question

In Java, abstract classes give the ability to define both concrete and abstract methods whereas interfaces only give the ability to implement abstract methods. I believe overriding methods in subclasses/implementations is possible in both cases, therefore, what is the real advantage of one over the other (interfaces vs abstract classes in Java)?

Answer 1

Interfaces are for when you want to say "I don't care how you do it, but here's what you need to get done."

Abstract classes are for when you want to say "I know what you should do, and I know how you should do it in some/many of the cases."

Abstract classes have some serious drawbacks. For example:

class House {  }  class Boat {  }  class HouseBoat extends /* Uh oh!! */ {     // don't get me started on Farmer's Insurance "Autoboathome" which is also a helicopter } 

You can get through via an interface:

interface Liveable {  }  interface Floatable {  }  class HouseBoat implements Liveable, Floatable {  } 

Now, abstract classes are also very useful. For example, consider the AbstractCollection class. It defines the default behavior for very common methods to all Collections, like isEmpty() and contains(Object). You can override these behaviors if you want to, but... is the behavior for determining if a collection is empty really likely to change? Typically it's going to be size == 0. (But it can make a big difference! Sometimes size is expensive to calculate, but determining whether something is empty or not is as easy as looking at the first element.)

And since it won't change often, is it really worth the developer's time to implement that method every... single... time... for every method in that "solved" category? Not to mention when you need to make a change to it, you're going to have code duplication and missed bugs if you had to re-implement it everywhere.

Answer 2

Interfaces are useful because Java doesn't have multiple inheritance (but you can implement as many interfaces as you like).

Abstract classes are useful when you need concrete behaviour from the base class.