On the static final keywords in Java

Question

According to the tutorial:

The static modifier, in combination with the final modifier, is also used to define constants. The final modifier indicates that the value of this field cannot change.

I would agree with this only if the types involved were primitive. With reference types, e.g. an instance of a class Point2D where its position attributes were not final (i.e., we could change its position), the attributes of this kind of variables such as public static final Point2D A = new Point2D(x,y); could still be changed. Is this true?

Answer 1

Yes, it can be changed. Only the references cannot be changed, but its internal fields can be. The following code shows it:

public class Final {
    static final Point p = new Point();
    public static void main(String[] args) {
        p = new Point(); // Fails
        p.b = 10; // OK
        p.a = 20; // Fails
    }
}

class Point {
    static final int a = 10;
    static int b = 20;
}

Groovy (an alternative JVM language) has an annotation called @Immutable, which blocks changing to a internal state of an object after it is constructed.