抽象類 (abstract)－Cedric's 學習備忘錄

1. 用abstract 修飾類時，則此類被稱之為抽象類

2. 含有抽象方法的類必須被聲明為抽象類，抽象類必須被繼承、抽象方法必須被重寫，否則子類就也要定義為抽象類及含有抽象方法，若子類繼承後沒有重寫方法，則出現錯誤 (Cat is not abstract and does not override abstract method enjoy() in Animal)

3. 抽象類不能被實例化，也就是不能以new產生新的物件，否則會出現錯誤 (Animal is abstract; cannot be instantiated)

4. 抽象方法只需被聲明，不需被實現

以下例子便可看出抽象類及方法的規則：

abstract class Animal {
   public String name;
   Animal(String name){
       this.name = name;
   }
   /*
   public void enjoy() {
       System.out.println("叫聲...");
   }
   */

   public abstract void enjoy();
}

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 Test {

   public static void main(String[] args) {

       Cat c = new Cat("Cat", "bule");
       Dog d = new Dog("Dog", "black");
       Bird b = new Bird("Bird", "green");
       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();

   }

}