The meaning of the interface

Old growth talk about what's the use of interfaces

The meaning of the interface existence:
1. When multiple implementation classes have the same method, using an interface reduces the amount of code
2. In the future need to add a method, directly in the interface to write, inherit the interface implementation class will tell you to add a method.
3. The interface is equivalent to a specification,
(a) When personal development, you write the interface, your own realistic implementation of the class, then there is no need to write interface.
(b) Team development, I wrote the interface, I suddenly have something to leave, you write the implementation class, you read the interface to know what to write. This time is very necessary.

The meaning of the interface