Intersecting Objects using Java

Question

Hullo everyone,
I've got a problem at the moment so I've decided to turn to these forums once again for some helpful insight as 2 + hours of Googling hasn't helped me in the slightest. I'm trying to create a mini-scale Katamari game (hopefully you're familiar with it, basically it's one ball rolling over other balls to grow in size) on Java and I've finally got all my graphics and move directions in place. Now though I can't seem to get the collision to work...
I'm trying to make the Katamari circle intersect with the little circle items on the playing field but currently the ball circle just ghosts over them. So basically I'm trying to get a circle to collide with other circles but I have no idea where to start. :S
My circle items contain int x, y and radius fields and have been automatically set to their respective sizes by the game.
I was thinking of working something out like:
boolean testIntersection(Item item)
if (distance between katamari ball and item <= radius of ball + radius of item)
return true;
else return false;
But whenever I put something like that in, it still doesn't detect collision. Any thoughts or help on this would be very much appreciated. :)

Answer 1

If you use object that implement Shape, then there is a contains and a collision implemented in the Shape interface.
Aside from that, you need to calculate from the center point of both your objects to get a true distance:
if your objects use an upper left corner for the drawing location and that is what your x and y represent, then you need to translate that to the center of your object. The distance between you objects can be given by:
sqrt((X2-X1)^2 + (Y2-Y1)^2)
Where X1, X2, Y1, and Y2 are center coordinates of your object.
Note this well always be a positive value.
The tolerance of your distances for an intersection would be R1+R2 so you will get the resulting conditional:
if((R1+R2)<sqrt((X2-X1)^2 + (Y2-Y1)^2) {
  //what ever collision code you want
}Please note, that you may want to include equality into your conditional and use the "<=" comparator, to include the condition that they "kiss".
Also please note that these values need to be implemented in double, rather than int to do away with errors introduced by integer math and when you do implement using double, then double introduces it's own set of errors due to incapability to absolutely represent values not inherently reproducible by the double type. To over come that you may want to implement using a delta value or "close enough:
{code}
if(delta < abs(((R1+R2)-sqrt((X2-X1)^2 + (Y2-Y1)^2)) {
//what ever collision code you want
{code}