Interface vs Abstract class

Interfaces

An interface is a contract: the guy writing the interface says, “hey, I accept things looking that way“, and the guy using the interface says “Ok, the class I write looks that way“.

An interface is an empty shell, there are only the signatures of the methods, which implies that the methods do not have a body. The interface can’t do anything. It’s just a pattern.
Implementing an interface consumes very little CPU, because it’s not a class, just a bunch of names, and therefore there is no expensive look-up to do. It’s great when it matters such as in embedded devices.

Abstract classes

Abstract classes, unlike interfaces, are classes. They are more expensive to use because there is a look-up to do when you inherit from them.

Abstract classes look a lot like interfaces, but they have something more : you can define a behavior for them. It’s more about a guy saying, “these classes should look like that, and they have that in common, so fill in the blanks!”.

e.g:

 

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