Java继承字段 [英] Java inheritance fields

问题描述

我无法理解以下输出。

我不知道为什么输出为10，我认为该行 A a = new B（）创建一个B类的新实例，我认为结果应为20

I don't know why the output is 10, I think the line A a = new B() creates a new instance of class B, I think the result should be 20

class A {
    int i = 10;
}

class B extends A {
    int i = 20;
}

public class MainClass {
    public static void main(String[] args) {
        A a = new B();

        System.out.println(a.i);
    }
}

为什么会这样工作..请解释。

Why this works like this .. please explain.

推荐答案

首先，参见 隐藏字段 （强调添加）

First, see Hiding Fields (emphasis added)

在一个类中，与超类中的字段同名的字段会隐藏超类的字段，即使它们的类型不同

Within a class, a field that has the same name as a field in the superclass hides the superclass's field, even if their types are different

换句话说，这不是继承，因为你实际上是在隐藏 A 的我在 B 后面 i 后面，你正在使用一个参考对象 A ，所以你得到它的字段。如果你做了 B b = new B（），你会看到 20 ，如预期的那样。

In other words, this isn't "inheritance" since you're actually hiding A's i behind B's i, and you are using a reference object of A, so you are getting its fields. If you did B b = new B(), you would see 20, as expected.

如果您期望真正的覆盖，请尝试使用方法。

If you expect true overrides, try using methods.

class A {
    public int get() { 
        return 10; 
    }
}

class B extends A {
    @Override 
    public int get() { 
        return 20; 
    }
}

参见

A a = new B();
System.out.print(a.get()); // 20

---

如果你真的想看到两者马上看这个例子。

---

If you really want to see both at once, see this example.

class A {
    int i = 10;
}

class B extends A {
    int i = 20;

    @Override 
    public String toString() { 
        return String.format("super: %d; this: %d", super.i, this.i);
    }
}

和

A a = new B();
System.out.print(a); // super: 10; this: 20

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