Priority Queue in Java using ArrayList

class PriorityQueue {

    private ArrayList maxHeap;

    public PriorityQueue () {
        maxHeap = new ArrayList();
        maxHeap.add(null);   //to fill element 0
    }

    public boolean empty () {
        return maxHeap.size() == 1;
    }

    public void insert (Comparable p) {
 if (p==null) return;
 maxHeap.add(p);
 int index = maxHeap.size()-1;
 swapWithParent(index);

    }
    
 private void swapWithParent(int index)
 {
  int parentindex = index/2;
  if (parentindex==0) return;
  if (p.compareTo(maxHeap.get(parentindex)) > 0)
  {
   //swap them
   Comparable p = maxHeap.get(index);
   maxHeap.set(index,maxHeap.get(parentindex));
   maxHeap.set(parentindex,p);
   swapWithParent(parentindex);
  }
  
  
 }

}