这里内部发生​​了什么？ [英] What happened internally here?

问题描述

class A
{
    int i=10;
    void show()
    {
        System.out.println("show()");
    }
}

class B extends A
{
    int j=20;
    void show1()
    {
        System.out.println("show1()");
    }
    public static void main(String ar[])
    {
        A a1=new B();//What happened internally here.please give me answer.
        a1.show();
        a1.show1();
    }
}

推荐答案

A a1=new B();//What happened internally here.please give me answer.
a1.show();
a1.show1();

发生的事情是你创建了 B的实例并为 A 类型的变量分配了引用。没关系，因为 B 实例是 A 。

What happened is that you created an instance of B and assigned the reference to an variable of type A. That's OK, because a B instance is a A.

在下一行中，您在 B 实例上调用了一个 A 方法。没关系。

In the next line you called one of the A methods on the B instance. That's OK.

在最后一行中，您尝试调用 B 方法。但由于静态类型 a1 是 A ，导致编译错误。但是，如果您编写了以下内容，它将编译并运行正常。

In the last line you attempted to call a B method. But since the static type of a1 is A that results in a compilation error. However, if you had written the following, it would have compiled and run just fine.

((B) a1).show1();

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