Java-21，多态的三个条件

package com.nyist;

class Animal {
  private String name;
  
  Animal(String name){
      this.name = name;
  }
  
  public void enjoy(){
    System.out.println("叫声......");
  }
  
}

class Cat extends Animal {
  private String eyesColor;
  Cat(String n,String c) {
      super(n); eyesColor = c;
  }
  
  public void enjoy() {
    System.out.println("猫叫声......");
  }
}

class Dog extends Animal {
  private String furColor;
  Dog(String n,String c) {
      super(n); 
      furColor = c;
  }
 
  public void enjoy() {
    System.out.println("狗叫声......");
  }
}

class Bird extends Animal {
     Bird() {
          super("bird");
     }
     
     public void enjoy() {
    System.out.println("鸟叫声......");
  }
}

class Lady {
    private String name;
    private Animal pet;
    Lady(String name,Animal pet) {
        this.name = name; this.pet = pet;
    }
    public void myPetEnjoy(){pet.enjoy();}
}

public class Test {
    public static void main(String args[]){
        Cat c = new Cat("catname","blue");
        Dog d = new Dog("dogname","black");
        Bird b = new Bird();
        Lady l1 = new Lady("l1",c);
        Lady l2 = new Lady("l2",d);
        Lady l3 = new Lady("l3",b);
        l1.myPetEnjoy();
        l2.myPetEnjoy();
        l3.myPetEnjoy();
    }
}

猫叫声......
狗叫声......
鸟叫声......