code: public boolean indexExists(int index) {
      assert index < 0 || 
      (index >= 0 && index <= mask) ||
      (index == mask + 1 && hasEmptyKey);
  
      return index >= 0; 
    }
docs: @see #indexOf
  
  @param index The index of a given key, as returned from {@link #indexOf}.
  @return Returns <code>true</code> if the index corresponds to an existing key
  or false otherwise. This is equivalent to checking whether the index is
  a positive value (existing keys) or a negative value (non-existing keys).