What are the necessary conditions for polymorphism to exist in Java?

Three necessary conditions for the existence of polymorphism:

(Recommended learning: java introductory tutorial)

• Inheritance

• Override

• The parent class reference points to the child class object

When When calling a method using polymorphism, first check whether the method exists in the parent class. If not, a compilation error occurs; if so, call the method of the same name in the subclass.

Benefits of polymorphism:

It allows the program to be well expanded and can handle objects of all classes universally.

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Test code:

public class Test {
    public static void main(String[] args) {
      show(new Cat());  // 以 Cat 对象调用 show 方法
      show(new Dog());  // 以 Dog 对象调用 show 方法
                
      Animal a = new Cat();  // 向上转型  
      a.eat();               // 调用的是 Cat 的 eat
      Cat c = (Cat)a;        // 向下转型  
      c.work();        // 调用的是 Cat 的 work
  }  
            
    public static void show(Animal a)  {
      a.eat();  
        // 类型判断
        if (a instanceof Cat)  {  // 猫做的事情 
            Cat c = (Cat)a;  
            c.work();  
        } else if (a instanceof Dog) { // 狗做的事情 
            Dog c = (Dog)a;  
            c.work();  
        }  
    }  }
 abstract class Animal {  
    abstract void eat();  }  
  class Cat extends Animal {  
    public void eat() {  
        System.out.println("吃鱼");  
    }  
    public void work() {  
        System.out.println("抓老鼠");  
    }  }  
  class Dog extends Animal {  
    public void eat() {  
        System.out.println("吃骨头");  
    }  
    public void work() {  
        System.out.println("看家");  
    }  }

Output result:

吃鱼
抓老鼠
吃骨头
看家
吃鱼
抓老鼠

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