从外部类访问内部类的私有实例变量 [英] Accessing private instance variable of inner class from outer class

问题描述

为什么此代码不起作用

public class BB
{
    private class A
    {
        private int x;
    }

    public static void main(String[] args)
    {
        A a = new A();
        a.x = 100;
        System.out.println(a.x);
    }
}

这段代码在起作用吗?

public class BB
{
    private class A
    {
        private int x;
    }

    static int y = 3;

    public static void main(String[] args)
    {
        BB b = new BB();
        b.compile();
        System.out.println("y = "+ y);
    }
    public void compile()
    {
        A a = new A();
        a.x = 100;
        System.out.println(a.x);
        System.out.println("y = "+ y);
    }
}

在第一个代码中，当我尝试通过内部类'a'的对象引用内部类'A'的实例变量'x'时，我收到一条错误消息，说我在静态使用内部类语境. 使用其他方法进行相同操作时没有错误.

In first code, When I am trying to refer to instance variable 'x' of inner class 'A' by an object of inner class 'a', I am getting an error saying that I'm using inner class in static context. There is no error while doing the same in some other method.

推荐答案

您的错误与字段访问无关.此行的编译失败:

Your error has nothing to do with field access. Compilation fails for this line:

A a = new A();

原因:没有封闭的实例，您无法实例化内部类，而这正是该代码行尝试执行的操作.您可以改写

Reason: you cannot instantiate an inner class without an enclosing instance, which is exactly what that line of code tries to do. You could write instead

A a = (new BB()).new A();

，它将提供一个封闭的实例内联.然后，您也可以访问私有字段.

which would provide an enclosing instance inline. Then you will be able to access the private field as well.

或者，只需将A类设置为static，这意味着它没有封闭的实例.

Alternatively, just make the A class static, which means it does not have an enclosing instance.

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