Question

I have no idea what immutable class should look like but am pretty sure this one is. Am I right? If I’m not please specify what should be added/removed.

import java.io.Serializable;

public class Triangle implements IShape, Serializable {
    private static final long serialVersionUID = 0x100;

    private Point[] points;

    public Triangle(Point a, Point b, Point c) {
        this.points = new Point[]{a, b, c};
    }

    @Override
    public Point[] getPoints() {
        return this.points;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) return false;
        if (this == obj) return true;
        if (getClass() != obj.getClass()) return false;
        Point[] trianglePoints = ((Triangle) obj).getPoints();
        for (int i = 0; i < points.length; i++){
            if (!points[i].equals(trianglePoints[i])) return false;
        }
        return true;
    }
}

Will this do the trick?

@Override
    public Point[] getPoints() {
        Point[] copyPoint = {
                new Point(points[0]),
                new Point(points[1]),
                new Point(points[2]),};
        return copyPoint;
    }

Point class:

import java.io.Serializable;

public class Point implements Serializable {
    private static final long serialVersionUID = 0x100;

    public int x;
    public int y;
    public int z;

    public Point(int x, int y, int z) {
        this.x = x;
        this.y = y;
        this.z = z;
    }

    public Point(Point that) {
        this.x = that.x;
        this.y = that.y;
        this.z = that.z;
    }

    public boolean equals(Object obj) { 
        // assume this is a typical, safe .equals implementation
        // that compares the coordinates in this instance to the
        // other instance
        return true;
    }
}

Answer 1

No, you can change what’s in the Points array. If you want to make it immutable, have the getter hand out a copy of the Points array, not the original.

try this:

Triangle triangle = new Triangle(a, b, c);
triangle.getPoints()[1] = null;
System.out.println(Arrays.toString(triangle.getPoints()));

Also Point needs to be immutable (as Nikita Rybak points out). For how to copy arrays see how to copy an array in Java.