/**
 * hashCode that combines three strings using xor.
 * @return a hash code value on the entire object.
 */
public int hashCode()
   {
   // this will not work well if some of the strings are equal.
   int result = string1.hashCode();
   result ^= string2.hashCode();
   result ^= string3.hashCode();
   // etc for all fields in the object
   return result;
   }