封闭类成员的内部类访问

Question

I will be amazed if anyone can answer this question. I am a beginner struggling immensely with the syntax and logic of nested classes in Java. If you run the following program, 'a' will print instead of 'b'. Why?

class MainClass
{
    public static void main(String[] args)
    {
        Outer OuterRefVar_a = new Outer('a');

        Outer OuterRefVar_b = new Outer('b');

        OuterRefVar_a.InnerTypeMember = OuterRefVar_a.new Inner();

        OuterRefVar_b.InnerTypeMember = OuterRefVar_a.InnerTypeMember;

        OuterRefVar_b.InnerTypeMember.set_innerChar_to_outerChar();

        System.out.println(OuterRefVar_b.InnerTypeMember.innerChar);
    }
}
class Outer
{
    char outerChar;

    Outer(char outerChar)
    {
        this.outerChar = outerChar;
    }

    class Inner
    {
        char innerChar;

        void set_innerChar_to_outerChar()
        {
            innerChar = outerChar;
        }
    }

    Inner InnerTypeMember;
}

Answer 1

That happens because while you have set the InnerTypeMember reference of object of A onto B..

OuterRefVar_b.InnerTypeMember = OuterRefVar_a.InnerTypeMember;

The inner object of A still has a reference to it's original Outer object A and will reference its member variables. Java implements inner classes by giving the object a secret reference to "Outer.this" which doesn't change simply by setting the InnerTypeMember on the other instance.

For example, if you had a InnerTypeMember variable within a completely different class, calling set_innerChar_to_outerChar() would still be expected to find Outer.outerChar on the object for which the inner class was original constructed.