Interfaces

Interfaces are in effect classes but they can have only abstract methods. No instance variables, no concrete methods, etc. Classes can implement interfaces. If they do they must make the abstract methods concrete.

Example. If Shape were an interface it could have an abstract method: public double getArea(double radius);.

If Circle implements the Shape interface it would need to make that method concrete:
public double getArea(double radius) { return Math.PI * radius * radius; }

Note that Shape couldn't be a class, as there is no formula for the area of a shape.

Why have interfaces? It's a way to have multiple inheritance without the big problems of multiple inheritance. Classes can extend only one class (superclass), but they can implement many interfaces.

There's much more that could be said, but those are the basics. Note that there are a number of good web sites that go into interfaces and inheritance in detail.