无法实例化内部类

Question

I'm learning how to use inner classes, but I'm encountering a troubling error when I try to compile it. I'm trying to see how inner and outer classes can use one another's variables and methods.

When I try to compile this code it says:

.../src/MyOuter.java:39: non-static variable inner cannot 
be referenced from a static context

Code:

public class MyOuter{
    private int x;

    public MyInner inner = new MyInner();

    public int getOuterX(){
        return x;
    }
    private void doStuff(){
        inner.go();
    }

    class MyInner{
        public int getInnerX(){
            return x;
        }
        void go(){
            x = 42;
        }
    }
    public static void main(String[] args) {
        MyOuter outer = new MyOuter();
        outer.doStuff();
        System.out.println("outer.x = " + outer.getOuterX());
        System.out.println("inner.x = " + inner.getInnerX());

    }
}

Thanks in advance for the help!

Answer 1

Since getInnerX() method is defined in MyInner class. You can't access it directly without the object of MyInner class .So change the line

 System.out.println("inner.x = " + inner.getInnerX());  

to

System.out.println("inner.x = " + outer.inner.getInnerX());

Answer 2

As was said, you first need to extract the inner variable before referencing it the static main method. Try something like the following:

{
    public static void main(String[] args) {
    MyOuter outer = new MyOuter();
    MyInner inner = outer.inner;
    outer.doStuff();
    System.out.println("outer.x = " + outer.getOuterX());
    System.out.println("inner.x = " + inner.getInnerX());
}

From Understanding Instance and Class Members :

Fields that have the static modifier in their declaration are called static fields or class variables. They are associated with the class, rather than with any object. Every instance of the class shares a class variable, which is in one fixed location in memory. Any object can change the value of a class variable, but class variables can also be manipulated without creating an instance of the class.

Since your inner variable is associated with an object, it cannot be referenced like a static variable might be. Were it static, it would be shared between all instance of 'MyOuter' and would be accessible in the manner you have tried there.