// Textbook fragment 09.04
  /** Returns the number of entries in the hash table. */
  public int size() { return n; }
  /** Returns whether or not the table is empty. */
  public boolean isEmpty() { return (n == 0); }
  /** Returns an iterable object containing all of the keys. */
  public Iterable<K> keys() {
    PositionList<K> keys = new NodePositionList<K>();
    for (int i=0; i<capacity; i++)
      if ((bucket[i] != null) && (bucket[i] != AVAILABLE))
        keys.addLast(bucket[i].getKey());
    return keys;
  }
  /** Helper search method - returns index of found key or -(a + 1),
   * where a is the index of the first empty or available slot found. */
  protected int findEntry(K key) throws InvalidKeyException {
    int avail = -1;
    checkKey(key);
    int i = hashValue(key);
    int j = i;
    do {
      Entry<K,V> e = bucket[i];
      if ( e == null) {
        if (avail < 0)
          avail = i;    // key is not in table
        break;
      }
      if (key.equals(e.getKey())) // we have found our key
        return i;       // key found
      if (e == AVAILABLE) {     // bucket is deactivated
        if (avail < 0)
          avail = i;    // remember that this slot is available
      }
      i = (i + 1) % capacity;   // keep looking
    } while (i != j);
    return -(avail + 1);  // first empty or available slot
  }
  /** Returns the value associated with a key. */
  public V get (K key) throws InvalidKeyException {
    int i = findEntry(key);  // helper method for finding a key
    if (i < 0) return null;  // there is no value for this key
    return bucket[i].getValue();     // return the found value in this case
  }