How to find all objects that are in collision with specified object.

While scripting bowling system I had to solve problem to monitor amount of ball in the arm, and control new ball providing.

Every time, when on the upper arm there is less than three ball I want to provide new ball. To find out how many balls is actually on arm, I was searching in unity for something like getListOfObjectsInWhichImInCollision(). What I found is Physics.OverlapSphere which "Returns an array with all colliders touching or inside the sphere." The point is, that sphere is not the shape that I could use for my problem so I made class for arm and override functions OnCollisionEnter/OnCollisionExit.

 class UpperArm extends MonoBehaviour {  
   private var ballAmount : int = 0;  
   private var ballTag = "Ball";  
   function OnCollisionEnter(c : Collision) {  
     if (itsBall(c)) {  
       ballAmount++;  
     }  
   }  
   function OnCollisionExit(c : Collision) {  
     if (itsBall(c)) {  
       ballAmount--;  
     }  
   }  
   function itsBall(c : Collision) {  
     return ((ballTag == c.transform.tag) ? true : false);  
   }    
 }

I needed only amount of balls on arm. If You need to have list of this objects: add a list variable and update it in onEnter/onExist function.