Help with simple method, Java

Question

This is really a silly question, but I've been staring at this problem for way too long and I just cant figure out what the problem is:

/**
     * public boolean overlap(int targetX, int targetY) {
     * Returns true if the target position is sufficient close to this ghost
     * If target position is 16 pixels or less in the x direction
     * and similarly in y direction, return true. Otherwise return false
     * 
     * @param targetX
     * @param targetY
     * @return
     */
    public boolean overlap(int targetX, int targetY){
        double x=this.getX();
        double y=this.getY();
        double deltaX=targetX-x;
        double deltaY=targetY-y;
        if(deltaX<=16 && deltaX>=0 && deltaY<=16 && deltaY>=0)
            return true;
        else
            return false;       
    }

This should work right? But it doesnt. If I run this test, it fails the assertTrue. (g1.x=100 and g1.y=1000)

    double theta = 2 * Math.PI * Math.random();
    int x = 100 + (int) (16 * Math.cos(theta));
    int y = 1000 + (int) (16 * Math.sin(theta));
    assertTrue(g1.overlap(x, y));

Does anyone see something I dont?

Answer 1

You're returning true if deltaX and deltaY are each between 0 and 16. But sin() and cos() aren't guaranteed to return positive numbers.

Answer 2

Based on the javadoc of the overlap method, you need to take the absolute value of targetX-x and targetY-y, and return true if both of those are less than or equal to 16.

So it would look like:

public boolean overlap(int targetX, int targetY){
    double x=this.getX();
    double y=this.getY();
    double deltaX = Math.abs(targetX-x);
    double deltaY = Math.abs(targetY-y);

    return (deltaX<=16 && deltaY<=16);       
}

Answer 3

Looks like deltaX and deltaY will be negative about half the time, and so your test will fail about 3/4 of the time.

Answer 4

If you want the distance regardless of direction shouldn't you have:

deltaX = Math.abs(targetX-x);
deltaY = Math.abs(targetY-y);

If your target is within 16px but to the left or above you would get a negative delta value and the method would return false.