Killer Game Programming in Java

Grouping the Boids

The FlockBehavior class is a flock manager and, consequently, must maintain a list of boids. The obvious data structure for the task is an ArrayList, but there's a subtle problem: boids may be deleted from the list, as when a prey boid is eaten. This can cause synchronization problems because of the presence of multiple behavior threads in the application.

For example, the PredatorBehavior thread may delete a prey boid from the prey list at the same time that PreyBehavior is about to access the same boid in the list. The solution is to synchronize the deleting and accessing operations so they can't occur simultaneously. This is the purpose of the BoidsList class:

    public class BoidsList extends ArrayList
    {
      public BoidsList(int num)
      {  super(num);  }
   
      synchronized public Boid getBoid(int i)
      // return the boid if it is visible; null otherwise
      { if (i < super.size())
          return (Boid)get(i);
        return null;
      }
   
     synchronized public boolean removeBoid(int i)
     // attempt to remove the i'th boid
     { if (i < super.size()) {
          super.remove(i);
          return true;
       }
       return false;
     }
   
    }  // end of BoidsList class

Another consequence of the dynamic change of the boids list is that code should not assume that the list's length stays the same. This means, for instance, that for loops using the list size should be avoided. If a for loop was utilized, then a change in the boids list may cause a boid to be processed twice, or skipped, because of its index position changing.