What is an abstract class

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Event address: CSDN 21-day Learning Challenge

I. What is an abstract class?
Class – a custom composite data type used to describe a set of things with common properties
Create a java class with the class keyword.
Java classes decorated with abstract are abstract classes.

Format: access modifier abstract class class name{}

Example:

public abstract class Hello{ }

Second, why use abstract classes?
The emergence of abstract classes reflects the refactoring of java. [Extracted repeated actions]
Abstract classes provide a general form that can be shared by subclasses, and subclasses handle this general form according to their actual situation.
For example: take a car to charge, taxis have a charging function [1 yuan/km], and buses have a charging function [1 yuan for the whole journey]
Subclasses of abstract classes are often the same thing.
4. What elements can be included in an abstract class?
Element instance variables, class variables, construction methods, instance methods, class methods, [abstract methods] in abstract classes.
Abstract method – abstract modification, a method without a method body.
An abstract method is the general form that an abstract class provides to subclasses.
An abstract class can have abstract methods or none at all.Ordinary java classes cannot have abstract methods.

For example

Three, the specific usage of abstract class
1. Abstract class cannot be new
2. Ordinary java class inherits abstract class, it needs to rewrite the abstract method in the abstract class, otherwise the ordinary java classThe class is changed to an abstract class.
3. Abstract classes can inherit other abstract classes without overriding abstract methods.
4. Create an abstract class object using the upcast object.
5. An abstract class object can access instance variables, class variables, construction methods, instance methods, class methods, [abstract methods] in the abstract class.
6. When an abstract class object accesses an abstract method, it actually accesses the abstract method after the subclass rewrites it.
7. When the method parameter in a common java class is an abstract class type, the upcast object can be passed, or it can be a subclass object of the abstract class.