Abstraction and interface

　　There can be private variables and methods in 1.abstract, but not in interfaces.

The former can have a concrete method of implementation, different from the interface, the interface method can not be implemented, so the private variables and methods will never be used.

2. Abstract methods can be used in abstract classes and interfaces.

3. Abstract classes can have data members and non-abstract methods in addition to abstract methods. While all methods in an interface must be abstract, data members can also be defined in an interface, but must be constants.

Abstraction and interface