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POhandling collisions - array look ups expensive
 

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    <p>I have a collision detection method in my android program to detect when two balls have collided and to calculate their new velocities. Every ball is stored in an array <code>Ball[] ballArray</code>. To handle collisions, I iterate through each ball, take it's center coordinates and calculate a surrounding region. If the center of any other ball lies within this region, then I check the two balls for a collision.</p> <p>My method is basically as follows</p> <pre><code>public void handleCollisions(){ int size = ballArray.length; for(int i = 0; i &lt; size; i++){ Ball ball = ballArray[i]; int centerX = ball.centerX; int centerY = ball.centerY; int radius = ball.radius; //calculate box around ball int lowXLimit = (centerX - radius - largestPossibleRadius); int highXLimit = (centerX + radius + largestPossibleRadius); int lowYLimit = (centerY- radius - largestPossibleRadius); int highYLimit = (centerY+ radius + largestPossibleRadius); for(int j = j+1; j &lt; size; j++){ Ball ball2 = ballArray[j]; int centerX2 = ball.centerX; int centerY2 = ball.centerY; int radius2 = ball.radius; //if ball2 is inside the possible collision region around ball1 if (centerX2 &gt; lowXLimit &amp;&amp; centerX2 &lt; highXLimit &amp;&amp; centerY2 &gt; lowYLimit &amp;&amp; centerY2 &lt; highYLimit) { //do calculations to determine new velocities } } </code></pre> <p>I was getting pretty poor performance from my code and ran a traceview. To my surprise, only around 5% of the execution time in this method is spent doing the collision resolution calculations (quite a number of floating point divisions, multiplications). In fact, around 60-70% of time in this method is simply accessing the balls from the ball array (lines <code>Ball ball = ballArray[i]</code> and <code>Ball ball2 = ballArray[j]</code>). </p> <p>I've tried using an <code>ArrayList</code> but that is even worse. Is there something I can do to speed up this code? perhaps another data structure? It seems like a really long time to me. Should I try reduce the number of member variables in <code>Ball</code>?</p>
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    1. COI wonder if it wouldn't be more efficient to declare ball and ball2 outside of the loops and only re-affecting at each iteration
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    2. COalso, I think linked list may be more efficient to loop through elements
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    3. CO`ball` and `ball2` are only references to existing Objects - not a slow operation (`new`). The difference to declaring those outside the loop and just updating the reference inside the loop should be close to zero. But try it - it's an interesting question.
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