Abstract class implements Interface

Question

为什么实现接口的抽象类不必实现接口的方法，而类却必须实现所有方法呢？

Answer 1

由于抽象类将其一个或多个方法的实现留给其扩展类，因此它可以通过实现其他接口继承它所继承的方法契约。

Answer 2

It is not necessary that class has to implements all the methods of an implemented interface. If class don't implement all the methods of an interface it can be declared as abstract class.

Answer 3

Abstract class, by definition, is a class that can have unimplemented methods. If an abstract class implements an interface, it is not bound to implement any of the the inherited methods. That doesn't mean you can't implement any of them. :D

Answer 4

Ultimately we have to create a concrete class. Only then we are going to create an instance and use it. Moreover after implementing an interface an abstract class won't become a concrete class but if a concrete class leaves a method unimplemented it must become an abstract class.