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。

以下是程序的代码：

public class Main {
    public static void main(String[] args) {
        Pet pet1 = new Dog();
        Pet pet2 = new Cat();
        pet1.setName("Buddy");
        pet2.setName("Kitty");
        System.out.println(pet1.getName() + ": " + pet1.speak());
        System.out.println(pet2.getName() + ": " + pet2.speak());
    }
}

class Pet {
    private String name;

    public Pet() {}

    public Pet(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String speak() {
        return "I am a pet";
    }
}

class Dog extends Pet {
    private int weight;

    public Dog() {}

    public Dog(String name, int weight) {
        super(name);
        this.weight = weight;
    }

    public int getWeight() {
        return weight;
    }

    public void setWeight(int weight) {
        this.weight = weight;
    }

    @Override
    public String speak() {
        return "Woof Woof";
    }
}

class Cat extends Pet {
    public Cat() {}

    @Override
    public String speak() {
        return "Miao Miao";
    }
}

运行结果为：

Buddy: Woof Woof
Kitty: Miao Miao

以下是增加了Dog类的构造函数，以及添加了getWeight和setWeight方法后的代码：

class Dog extends Pet {
    private int weight;

    public Dog() {}

    public Dog(String name, int weight) {
        super(name);
        this.weight = weight;
    }

    public Dog(int weight) {
        this.weight = weight;
    }

    public int getWeight() {
        return weight;
    }

    public void setWeight(int weight) {
        this.weight = weight;
    }

    @Override
    public String speak() {
        return "Woof Woof";
    }
}

以下是重写了Dog类的speak()方法和Cat类的speak()方法后的代码：

class Dog extends Pet {
    private int weight;

    public Dog() {}

    public Dog(String name, int weight) {
        super(name);
        this.weight = weight;
    }

    public Dog(int weight) {
        this.weight = weight;
    }

    public int getWeight() {
        return weight;
    }

    public void setWeight(int weight) {
        this.weight = weight;
    }

    @Override
    public String speak() {
        return "Woof Woof";
    }
}

class Cat extends Pet {
    public Cat() {}

    @Override
    public String speak() {
        return "Miao Miao";
    }
}

以下是在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()方法的代码：

public class Main {
    public static void main(String[] args) {
        Pet pet1 = new Dog();
        Pet pet2 = new Cat();
        pet1.setName("Buddy");
        pet2.setName("Kitty");
        System.out.println(pet1.getName() + ": " + pet1.speak());
        System.out.println(pet2.getName() + ": " + pet2.speak());

        // 类型多态
        Pet[] pets = new Pet[2];
        pets[0] = new Dog();
        pets[1] = new Cat();

        for (Pet pet : pets) {
            System.out.println(pet.speak());
        }
    }
}

输出结果为：

Buddy: Woof Woof
Kitty: Miao Miao
Woof Woof
Miao Miao

结果正确，证明类型多态的方式可以实现Pet对象分别调用Dog和Cat的speak()方法。