Making a Class Immutable

Question

I am learning about immutable Objects. I have to make following class immutable. Did i do it right?

import java.awt.Point;
public class MyImmutablePoint {
    Point point;

    public MyImmutablePoint(Point point) {
        super();
        this.point = point;
    }

    public MyImmutablePoint() {
        this (new Point (0,0));
    }

    public Point getPoint() {
        return point;
    }

    public void setPoint(Point point) {
        this.point = point
    }
}

"Immutable" Class:

public final class MyImmutablePoint {
    private final Point point;

    public MyImmutablePoint(Point point) {
        this.point = point;
    }

    public MyImmutablePoint() {
        this (new Point (0,0));
    }

    public Point getPoint() {
        return point;
    }   
}

Iam not sure about the toString method though. and maybe returning an object like Point can be modified as well like an array but not sure

Answer 1

No

final Point p = new Point(0,0);
final ImmutablePoint ip = new ImmutablePoint(p);

Two examples:

//change the original Point passed in
p.x = 10
//use the getter and change the Point
ip.getPoint().x = 10

So, first you need to create a defensive copy of the Point taken in the constructor:

public MyImmutablePoint(Point point) {
    this.point = new Point(point);
}

Then you need to create a defensive copy of the Point returned from the getter:

public Point getPoint() {
    return new Point(point);
}

This all leads me to suggest that it would probably be better not to expose the internal point at all:

public final class MyImmutablePoint {

    private final Point point;

    public MyImmutablePoint(Point point) {
        this.point = new Point(point);
    }

    public MyImmutablePoint() {
        this.point = new Point (0,0);
    }

    public int getX() {
        return point.x;
    }

    public int getY() {
        return point.y;
    }
}

Further format your code and order your members.

Answer 2

No it is not immutable. Point can still be modified by the creator of MyImmutablePoint. Ex:

    Point point = new Point(1, 1);
    MyImmutablePoint immutablePoint = new MyImmutablePoint(point);
    point.setLocation(0, 0);