How to understand java polymorphism

In Java, polymorphism is the ability of the same behavior to have multiple different manifestations or forms; polymorphism is the same interface, using different instances to perform different operations. The advantages of polymorphism: 1. Eliminate coupling relationships between types; 2. Replacement; 3. Extensibility; 4. Interface; 5. Flexibility; 6. Simplification.

The operating environment of this tutorial: windows7 system, java8 version, DELL G3 computer.

Polymorphism Overview

1. Polymorphism is the third major feature of object-oriented after encapsulation and inheritance.

2. PolymorphismPractical meaningUnderstanding:

• Real things often take on multiple forms, such as students , students are a kind of people, then a specific classmate Zhang San is both a student and a person, that is, there are two forms.                                                              The language of objects can also describe multiple forms of a thing. If the Student class inherits the Person class, a Student object is both a Student and a Person.

• 3. Polymorphism is reflected in

  The parent class reference variable can point to the subclass object

  .

5. Definition and usage format of polymorphismDefinition format:Parent class type variable name=new subclass type();

Polymorphism is the ability of the same behavior to have multiple different manifestations or forms.

• Replaceability

• Extensibility

• Interface

##Flexibility
Simplification
8. Three necessary conditions for the existence of polymorphism
Inheritance

Rewrite
The parent class reference points to the child class object:
• Parent p = new Child();

• # #Characteristics of members in polymorphism

  1. Polymorphic member variables: Look on the left when compiling and running

Fu f=new Zi();
System.out.println(f.num);//f是Fu中的值，只能取到父中的值

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Fu f1=new Zi();
System.out.println(f1.show());//f1的门面类型是Fu,但实际类型是Zi,所以调用的是重写后的方法。

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instanceof keyword

Use case:

        Fu f1=new Zi();
        Fu f2=new Son();
        if(f1 instanceof Zi){
            System.out.println("f1是Zi的类型");
        }
        else{
            System.out.println("f1是Son的类型");
        }

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Polymorphic transformation

Polymorphism The transformation is divided into two types: upward transformation and downward transformation.

Upward transformation: polymorphism itself is a process of upward transformation

• Applicable scenarios: When there is no need to face subclass types, the corresponding operations can be completed by improving scalability or using the functions of the parent class.
Downward transformation: A subclass object that has been upwardly transformed can use the format of forced type conversion to convert the parent class reference type to the subclass reference type

Polymorphic case:

)

package day0524;

public class demo04 {
    public static void main(String[] args) {
        People p=new Stu();
        p.eat();
        //调用特有的方法
        Stu s=(Stu)p;
        s.study();
        //((Stu) p).study();
    }
}
class People{
    public void eat(){
        System.out.println("吃饭");
    }
}
class Stu extends People{
    @Override
    public void eat(){
        System.out.println("吃水煮肉片");
    }
    public void study(){
        System.out.println("好好学习");
    }
}
class Teachers extends People{
    @Override
    public void eat(){
        System.out.println("吃樱桃");
    }
    public void teach(){
        System.out.println("认真授课");
    }
}

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What is the result of running the project?

package day0524;
public class demo1 {
    public static void main(String[] args) {
        A a=new A();
        a.show();
        B b=new B();
        b.show();
    }
}
class A{
    public void show(){
        show2();
    }
    public void show2(){
        System.out.println("A");
    }
}
class B extends A{
    public void show2(){
        System.out.println("B");
    }
}
class C extends B{
    public void show(){
        super.show();
    }
    public void show2(){
        System.out.println("C");
    }
}