Summary of abstract classes and interfaces

Expression form of class

Java code

public class A{}

Characteristics of class:
1) The attributes in the class can be member attributes or class attributes
2) The methods in the class must be A method with a method body can either be a member method or a class method
3) Each class must have at least one constructor. The default is a parameterless constructor. You can call the constructor to create an object
A class Only one parent class can be inherited, single root inheritance

Java code

public class A{}

public class B extends A{}

Abstract class
Expression form:

Java code

public abs tract class A{}

Characteristics of abstract classes:
1) The attributes in abstract classes are the same as classes, they can be member attributes or class attributes
2) Methods in abstract classes can be abstract methods or concrete Member methods of ) Abstract class is used to act as a parent class for subclasses to inherit and extend

A class can only inherit one abstract class

Interface (interface)
Expression form:


Java code

public interface A{}

Characteristics of interfaces:

1) The properties in the interface must be public constants (must be assigned initial values)

2) The properties in the interface are public static final by default and must be public static final
3) Methods in the interface They must all be abstract methods
4) The methods in the interface default to public abstract and must be public abstract
5) The access qualifier of the methods in the interface defaults to public and must be public
6) The interface has no constructor and cannot be created Object (that is, you cannot create a new object)
7) The interface is used to act as a parent class to extend the subclass

There is also the implementation of the interface:


Java code

public class A implementations B{}

 Classes can implement multiple interfaces

Java code

public interface c{}

public interface d{}

public class E implementations c,d{}

Note: To implement an interface, a class must implement [ Rewrite] All abstract methods in the interface

When overriding inherited methods/implementing methods in the interface, the visible scope of the method cannot be reduced

A class can first inherit a class and then implement multiple interfaces


Java code

public class F extends A implementations c,d{}