理解Java中的继承概念 [英] Understanding the concept of inheritance in Java

问题描述

我只是刷新了java的oops功能。所以，我对继承概念有点困惑。为此，我有以下示例代码：

I am just refreshing the oops features of the java. So, I have a little confusion regarding inheritance concept. For that I have a following sample code :

class Super{
    int index = 5;
    public void printVal(){
        System.out.println("Super");
    }
}
class Sub extends Super{
    int index = 2;
    public void printVal(){
        System.out.println("Sub");
    }
}
public class Runner {
    public static void main(String args[]){
        Super sup = new Sub();
        System.out.println(sup.index+",");
        sup.printVal();
    }
}

现在上面的代码输出为： 5，Sub。

Now above code is giving me output as : 5,Sub.

这里，我们重写了printVal（）方法，因此可以理解它只是访问子类方法。

Here, we are overriding printVal() method, so that is understandable that it is accessing child class method only.

但是我无法理解为什么它从超级类中访问x的值...

But I could not understand why it's accessing the value of x from Super class...

提前致谢... ..

Thanks in advance....

推荐答案

对象有类型，变量有类型。因为你把：

Objects have types, and variables have types. Because you put:

Super sup = new Sub();

现在你有一个变量 sup 的类型超级，指的是 Sub 类型的对象。

Now you have a variable sup of type Super which refers to an object of type Sub.

当您在对象上调用方法时，将根据对象的类型选择运行的方法，这就是它打印Sub而不是Super的原因。

When you call a method on an object, the method that runs is chosen based on the type of the object, which is why it prints "Sub" instead of "Super".

当您访问对象中的字段时，将根据变量的类型选择字段，这就是您获得5的原因。

When you access a field in an object, the field is chosen based on the type of the variable, which is why you get 5.

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