Java中的多态性:为什么我们将父引用设置为子对象? [英] Polymorphism in java: Why do we set parent reference to child object?

问题描述

我想了解为子对象设置父引用的用例. 示例:狗类扩展了动物类. (请注意，没有接口) 我通常会这样创建一个Dog对象:

I want to understand the use-case of setting a parent reference to a child object. Example: Dog class extends Animal class. (No interfaces, mind it) I would normally create an object of Dog like this:

Dog obj = new Dog();

现在，由于Dog是Animal的子类，因此它已经可以访问Animal的所有方法和变量.然后，这有什么不同:

Now, since Dog is a subclass of Animal it already has access to all of Animal's methods and variables. Then, what difference does this make:

Animal obj = new Dog(); 

请提供一个正确的用例，并附上其使用的代码段.请不要提供有关多态性"或接口编码"的理论文章！

Please provide a proper use-case with an code snippet of it's use. No theoretical articles about 'Polymorphism' or 'Coding to interfaces' please!

代码:

public class Polymorphism {
    public static void main(String[] args){
        Animal obj1 = new Dog();
        Dog obj2 = new Dog();
        obj1.shout(); //output is bark..
        obj2.shout(); //output is bark..        
    }   
}

class Animal{
    public void shout(){
        System.out.println("Parent animal's shout");
    }       
}

class Dog extends Animal{
    public void shout(){
        System.out.println("bark..");
    }
}

class Lion extends Animal{
    public void shout(){
        System.out.println("roar..");
    }
}

class Horse extends Animal{
    public void shout(){
        System.out.println("neigh");
    }
}

在两种情况下输出都是相同的.那么为什么我们将父引用设置为子对象呢?

Output is the same for both the cases. Then why do we set parent reference to child object?

推荐答案

好的.我想我得到了答案.

Okay. I think I got my answer.

public class Polymorphism {
    public static void main(String[] args){
        Animal obj1 = new Horse();
        Horse obj2 = new Horse();

        obj1.shout();    //output is neigh..
        obj2.shout();    //output is neigh..
        obj1.winRaces(); /*But this is not allowed and throws compile time error, 
                           even though the object is of Animal type.*/ 
    }   
}

class Animal{
    public void shout(){
        System.out.println("Parent animal's shout");
    }       
}

class Horse extends Animal{
    public void shout(){
        System.out.println("neigh..");
    }
    public void winRaces(){
        System.out.println("won race..");
    }
}

因此，当我们将父级引用用于子类对象时，我们无法使用该对象访问子类中的任何特定方法(父类中不存在).

So, when we use parent reference for child class object, we cannot access any specific methods in child class (that are not present in parent class) using that object.

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