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

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

为什么这段代码不起作用？

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); } }

虽然这段代码有用吗？

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，当我试图通过内部类'a'的对象引用内部类'A'的实例变量'x'时，我收到一个错误，说我在静态上下文中使用内部类。 在其他方法中执行相同操作时没有错误。

Why isn't this code working

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); } }

while this code is working?

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); } }

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.

最满意答案

您的错误与字段访问无关。 此行的编译失败：

A a = new A();

原因：您无法在没有封闭实例的情况下实例化内部类，这正是该行代码尝试执行的操作。 你可以改写

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

这将提供内联的封闭实例。 然后您也可以访问私有字段。

或者，只需使A类static ，这意味着它没有封闭的实例。

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

A a = new A();

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();

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

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