多型 ( polymorphism)－Cedric's 學習備忘錄

構成多型的條件有三項，如下：

1. 要有繼承 (extends)

2. 一定有重寫 (override) 父類方法

3. 父類引用指向子類物件

當三個條件滿足，當你調父類中被重寫的方法時，實際是在調用new出來的子類物件方法，換言之，當你重寫父類方法時，你new出一個子類物件後，調用的就是子類物件中重寫的方法

可參考以下程式碼：

class Animal {
   public String name;
   Animal(String name){
       this.name = name;
   }

   public void enjoy() {
       System.out.println("叫聲...");
   }
}

class Cat extends Animal {
   public String eyesColor;
   Cat(String n, String c) {
       super(n);
       this.eyesColor = c;
   }

   public void enjoy() {
       System.out.println("貓叫聲...");
   }
}

class Dog extends Animal {
   public String furColor;
   Dog (String n, String f){
       super(n);
       this.furColor = f;
   }

   public void enjoy() {
       System.out.println("狗叫聲...");
   }
}

class Bird extends Animal {
   public String furColor;
   Bird (String n, String f) {
       super(n);
       this.furColor = f;
   }

   public void enjoy() {
       System.out.println("鳥叫聲...");
   }
}

class Lady {
   protected String name;
   private Animal pet;

   Lady (String name, Animal pet) {
       this.name = name;
       this.pet = pet;
   }

   public void myPetEnjoy() {
       pet.enjoy();
   }
}

public class TestPolymorphism {

   public static void main(String[] args) {

       Cat c = new Cat("Cat", "bule");
       Dog d = new Dog("Dog", "black");
       Bird b = new Bird("Bird", "green");a
       Lady l1 = new Lady("Cindy", c);
       Lady l2 = new Lady("阿美", d);
       Lady l3 = new Lady("小花", b);

       System.out.print(l1.name + "的寵物發出");
       l1.myPetEnjoy();
       System.out.print(l2.name + "的寵物發出");
       l2.myPetEnjoy();
       System.out.print(l3.name + "的寵物發出");
       l3.myPetEnjoy();

   }

}