Interfaces and abstract classes

The same thing ：

1、 Interfaces and abstract classes cannot be instantiated

2、 Can contain abstract methods

3、 Are at the top of the inheritance tree

Difference ：

1、 Multiple implementation 、 Single implementation

2、 Only static constants can be defined in an interface （public static final）, Ordinary member variables can be defined in abstract classes , You can also define static constants

3、 There are no constructors in the interface , Abstract classes can contain constructors , But the constructor of abstract classes is not used to create objects , Instead, let its subclass call these constructors to complete the initialization of the abstract class

4、 stay java8 Before , The interface contains only abstract methods , There is no way to achieve , Only the definition of method （java8 It is allowed to add static methods and default methods in the interface ）; Abstract classes can contain ordinary methods , It can also contain abstract methods

5、java8 Before , The methods in the interface are automatically public; The methods in an abstract class can be public、 default 、protected etc.

6、 The use of interfaces is more oriented towards certain functions , Abstract classes emphasize ownership , Such as A is a B

7、 If the implementation class implements an interface , Then it needs to implement all the abstract methods in this interface ; When subclasses inherit an abstract class , If it fails to implement all the abstract methods in the abstract class , So subclasses are also abstract classes , Still cannot be instantiated