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.

Answer 2

Correct, it can still be changed. The "static final", in this case, refers to the reference itself, which cannot be changed. However, if the object that it is references is mutable, then the object that it references can be changed.

An immutable object, such as a String, will be a constant.