Java Basics: Understanding and Application of Polymorphism

Overview of polymorphism

1. Polymorphism is the third major object-oriented method after encapsulation and inheritance. characteristic.

2. Understanding the meaning of polymorphic reality:

Real things often take on multiple forms, such as students. Students are a type of human being. A specific classmate Zhang San is both a student and a human being, that is, he appears in two forms.

As an object-oriented language, Java 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 fact that parent class reference variables can point to subclass objects.

4. Prerequisite: There must be a child-parent relationship.

Note: When calling a method using a polymorphic parent class reference variable, the overridden method of the subclass will be called.

5. Polymorphic definition and usage format

Definition format: parent class type variable name=new subclass type();

Characteristics of members in polymorphism

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

Fu f=new Zi();

System.out.println(f.num);//f is the value in Fu, and can only get the value in the parent

2. Polymorphic member method: look on the left when compiling, and on the right when running

Fu f1=new Zi();

System.out.println(f1.show());//The facade type of f1 is Fu, but the actual type is Zi, so The overridden method is called.

instanceof keyword

Function: Used to determine whether an object belongs to a certain data type.

Note: The return type is Boolean

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");
}

Polymorphic transformation

Polymorphic transformation points There are two types of upward transformation and downward transformation

Upward transformation: polymorphism itself is a process of upward transformation

Usage format: parent class type variable name=new subclass type ();

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 forced type conversion format to convert the parent class reference type to the subclass reference type.

Using format: subclass type Variable name = (subclass type) Variable of parent class type;

Applicable scenario: When you want to use the unique functions of the subclass.

Polymorphic case:

Example 1:

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");
    }
}

Example 2:

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");
    }
}

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