Java - 向上转发和向下转发 [英] Java - Upcasting and Downcasting

问题描述

我知道在stackoverflow中有很多文章/问题描述了Java中的向上转换和向下转换。我知道什么是向上倾斜和向下倾斜。但我的问题并不是特别的。

I Knew there are plenty of articles/questions in stackoverflow describing about upcasting and downcasting in Java. And I knew what is upcasting and downcasting. But my question is not specfic to that.

Upcasting - 从子节点转换为父节点 - 编译器需要注意。不需要演员阵容

向下转换 - 从父母到孩子的转换 - 需要明确演员？

Upcasting - Conversion from child to parent - Compiler takes care. No cast is required
Downcasting - Conversion from parent to child - Explicit cast is required

public class Animal {

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

    public void getDogName(){
        System.out.println("Dog");
    }
}

public static void main(String[] args) {

        Dog d = new Dog();
        Animal a = d; // Upcasting
        a.getAnimalName();

        Animal vv = new Dog();
        Dog cc = (Dog)vv;//DownCasting
        cc.getAnimalName();
        cc.getDogName();

如果你查看 Animal 和 Dog 类，每个都有自己的方法，如 getAnimalName（）和 getDogName（）。因此 Dog extends Animal （is-a relationship），所以我们可以在派生类（Subclass）中使用基类（Super Class）方法


现在考虑主方法中的下面一段代码，

所以我在这里创建一个 Dog 对象w.rt 动物。所以我只能访问 Animal 属性（方法）

If you look into the Animal and Dog class, each are having their own methods like getAnimalName() and getDogName(). Hence Dog extends Animal(is-a relationship), so we can use the base class(Super Class) methods in the derived class(Subclass)

Consider the below piece of code in the Main Method now,
So here I'm creating a Dog object w.rt Animal. So I can be able to access only the Animal properties(Methods)

Dog d = new Dog();
Animal a = d; // Upcasting
a.getAnimalName();<br>
O/P : Parent Animal<br><br>

现在让我们说，我想将基类方法覆盖到派生类中

/ p>

Now Let's say, I would like to override the base class methods into the derived class

public class Dog extends Animal{

    @Override
    public void getAnimalName(){
        System.out.println("Parent Animal overridden here");
    }
    public void getDogName(){
        System.out.println("Dog");
    }
}<br>

在主要方法中，

And in Main Method,

Dog d = new Dog();
Animal a = d; // Upcasting
a.getAnimalName();<br>
O/P : Parent Animal overridden here<br><br>

即使我正在创建 Dog object wrt Animal ，但是这里打印的基类方法在dervied类中被重写。

Even though I'm creating a Dog object w.r.t Animal, but here it is printing the base class method which is overridden in the dervied class.

 O/P : Parent Animal overridden here<br>

想知道为什么它会像这样。这是因为覆盖？

Wondering why it behaves like this. Is this becasue of the override?

请提供您宝贵的意见。

推荐答案

Animal a = d; 

此行将使您的Animal对象'a'指向Dog类的实例（Dog d =新狗（）;）。因此，当您调用该函数时，它将调用类dog中的函数。

this line will make your Animal object 'a' point to the instance of Dog class (Dog d = new Dog();). Therefore when you call the function it will invoke the function in class dog.

您实际创建了类Dog的实例 Dog d = new Dog（ ）; 。然后你正在创建一个类Animal的对象，并使它指向类的实例Dog Animal a = d;

You actually created an instance of class Dog Dog d = new Dog();. Then you are making an object of class Animal and making it point to the instance of class Dog Animal a = d;

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