/** 
   Searches for the first occurrence of the given argument, 
   testing for equality using the equals method. 
   @param elem an object. 
   @return the index of the first occurrence 
   of the argument in this list; returns -1 if 
   the object is not found. 
*/ 
public int indexOf(Object elem) 
{ 
   if (elem == null) 
   { 
      for (int i = 0; i < size; i++) 
         if (elementData[i] == null) return i; 
   } 
   else 
   { 
      for (int i = 0; i < size; i++) 
         if (elem.equals(elementData[i])) return i; 
   } 
   return -1; 
}