1)首先阅读下列程序并写出运行结果，然后编译、执行以验证; 2) 增加Dog类的构造函数，使之可以初始化Dog类中的weight值，并实现构造函数重载； 3)在Dog类中添加方法getWeight、setWeight以存取weight值； 4) 重写Dog类对象的speak()方法，使其返回“Woof Woof”字符串； 5)从Pet类派生Cat类，并且重写speak()方法返回值为”Miao Miao”; 6)在Main方法中，通过类型多态的方式实现Pet对象分别调用Dog和Cat的speak()方法，分析输出结果是否正确。 class Pet { protected String name; public Pet(String n) { name = n; } public String getName() { return name; } public String move() { return "run"; } public String speak() { return ""; } public String toString() { return "My pet " + name; } } class Dog extends Pet { protected int weight; public Dog(String s) { super(s); } } class PetTest { public static void main(String[] args) { Pet myPet = new Pet("George"); Dog myDog = new Dog("Spot"); System.out.println(myPet.toString() + "\n" + "Speak: " + myPet.speak() + "\n" + myPet.move() + " " + myPet.getName() + "\n"); System.out.println(myDog.toString() + "\n" + "Speak: " + myDog.speak() + "\n" + myDog.move() + " " + myDog.getName() + "\n"); } }

运行结果如下：

My pet George
Speak: 
run George

My pet Spot
Speak: 
run Spot

增加Dog类的构造函数，使之可以初始化Dog类中的weight值，并实现构造函数重载:

class Dog extends Pet {
    protected int weight;
    public Dog(String s) {
        super(s);
    }
    public Dog(String s, int w) {
        super(s);
        weight = w;
    }
}

在Dog类中添加方法getWeight、setWeight以存取weight值:

class Dog extends Pet {
    protected int weight;
    public Dog(String s) {
        super(s);
    }
    public Dog(String s, int w) {
        super(s);
        weight = w;
    }
    public int getWeight() {
        return weight;
    }
    public void setWeight(int w) {
        weight = w;
    }
}

重写Dog类对象的speak()方法，使其返回“Woof Woof”字符串:

class Dog extends Pet {
    protected int weight;
    public Dog(String s) {
        super(s);
    }
    public Dog(String s, int w) {
        super(s);
        weight = w;
    }
    public int getWeight() {
        return weight;
    }
    public void setWeight(int w) {
        weight = w;
    }
    public String speak() {
        return "Woof Woof";
    }
}

从Pet类派生Cat类，并且重写speak()方法返回值为”Miao Miao”:

class Cat extends Pet {
    public Cat(String s) {
        super(s);
    }
    public String speak() {
        return "Miao Miao";
    }
}

在Main方法中，通过类型多态的方式实现Pet对象分别调用Dog和Cat的speak()方法，分析输出结果是否正确:

public static void main(String[] args) {
    Pet myPet = new Pet("George");
    Dog myDog = new Dog("Spot");
    Cat myCat = new Cat("Tom");
    Pet[] pets = {myPet, myDog, myCat};
    for (Pet pet : pets) {
        System.out.println(pet.toString() + "\n" + "Speak: " + pet.speak() + "\n" + pet.move() + " " + pet.getName() + "\n");
    }
}

输出结果如下:

My pet George
Speak: 
run George

My pet Spot
Speak: Woof Woof
run Spot

My pet Tom
Speak: Miao Miao
run Tom

可以看到，输出结果是正确的，实现了Pet对象通过类型多态的方式分别调用Dog和Cat的speak()方法。