non-static inner class

By default, an inner class is non-static:

public class A
{
    int y;

    public class B
    {
int x;

void f () {}
    }
}

This fragment defines the class A which contains a non-static inner class B.

A non-static inner class can be instantiated only inside a non-static method of the outer class. This is because every instance of a non-static inner class must be associated with an instance of the outer class. In a sense, every instance of a non-static inner class exists ``inside'' an instance of the outer class. A single instance of the outer class may have associated with it more than one instance of the inner class.

Because an instance of a non-static inner class has an associated instance of the outer class, the methods of the inner class can access directly any of the members (fields or methods) of the outer class instance. For example, the f method defined above can access both x and y directly.

The Java keyword this  can be used in a non-static method to refer to the current object instance. Thus in the method f, this refers to an instance of the inner B class. Every non-static inner class is associated with an instance of the outer class. To access the outer class instance inside the method f we write A.this.