2022-07-26 group 4 abstract classes and interfaces

abstract

abstract
Abstract methods do not have braces , There is no method body
The significance of abstract methods
Fundamental significance , Appointment ： Custom rules
Agreed return value , Access right , parameter list ,, Need to rewrite , Restart the first method
Convention over configuration , Configuration is greater than encoding
Abstract methods must be in an abstract class
    When we define an abstract class , What structures can there be in this abstract class
        attribute , Member method , Constructors , Abstract method , Constant
     Can there be no abstract methods in an abstract class
        Sure
    Abstract classes cannot be instantiated
        Abstract classes cannot be created objects
The significance of the existence of constructors of abstract classes ：
    In order to agree that the constructor of a subclass must match the parent class
If a class inherits an abstract class , You must override all abstract methods of this abstract class
abstract What can be modified ？
    class 、 Method .
summary ： The abstract method must be public and protected
    Abstract classes cannot be instantiated directly , You need to rely on subclasses to handle it in an upward transformation way
    Abstract classes must have subclasses , Use extends Inherit , A class can only inherit one abstract class
    Subclass （ If it's not an abstract class ） You must rewrite all abstract methods in the class
        （ If the subclass does not implement the abstract method of the parent class , Subclasses must be defined as abstract）

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Interface

More advanced abstractions than abstract classes
Abstract classes are also more advanced than classes
Statement ： Use interface keyword
Only abstract methods can be defined in an interface , Cannot have attributes , There can't be a way .
( You can put static constants and static methods 、 The default method )
Characteristics of the structure in the interface ：
    Methods in the interface default to public
The structure in the interface must be public
Interfaces can be implemented
If a class implements an interface , It needs to rewrite all methods of the interface
The meaning of interfaces ：
Appointment
java There are multiple inherited , Occurs between interfaces
There is single inheritance between classes

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Anonymous implementation class

 public class Ch01 {
    public static void main(String[] args) {
        Abstr01 abstr01=new Abstr01() {
            public void show(){
                System.out.println(" rewrite show Method ");
            }
        };
        abstr01.show();
    }
}