Java SE 21st----Abstract class

1. Abstract class: A class that uses the abstract keyword modifier is called an abstract class, and an abstract class cannot be instantiated , that is, an object of an abstract class cannot be new.

2. Abstract method: The method using the Abstract keyword modification is called an abstract method, there is no declaration of implementation (no curly brace public abstract void Method ();)

abstract methods need to be defined in an abstract class. In contrast to abstract methods, the previously defined methods are called concrete methods (with declarations, implementations).

3. If a class contains an abstract method, then the class must be an abstract class.

4. If a class is an abstract class, then the class can contain concrete methods (declarations are implemented).

5. If a class contains abstract methods, then the class must be declared abstract, that is, the class must be an abstract class, reproduction if a class is an abstract class, then the class can contain abstract methods and concrete methods.

Java SE 21st----Abstract class