// Textbook fragment 09.05
  /** Put a key-value pair in the map, replacing previous one if it exists. */
  public V put (K key, V value) throws InvalidKeyException {
    int i = findEntry(key); //find the appropriate spot for this entry
    if (i >= 0) //  this key has a previous value
      return ((HashEntry<K,V>) bucket[i]).setValue(value); // set new value
    if (n >= capacity/2) {
      rehash(); // rehash to keep the load factor <= 0.5
      i = findEntry(key); //find again the appropriate spot for this entry
    }
    bucket[-i-1] = new HashEntry<K,V>(key, value); // convert to proper index
    n++;
    return null;        // there was no previous value
  }
  /** Doubles the size of the hash table and rehashes all the entries. */
  protected void rehash() {
    capacity = 2*capacity;
    Entry<K,V>[] old = bucket;
    bucket = (Entry<K,V>[]) new Entry[capacity]; // new bucket is twice as big
    java.util.Random rand = new java.util.Random();
    scale = rand.nextInt(capacity-1) + 1;       // new hash scaling factor
    shift = rand.nextInt(capacity);             // new hash shifting factor
    for (int i=0; i<old.length; i++) {
      Entry<K,V> e = old[i];
      if ((e != null) && (e != AVAILABLE)) { // a valid entry
        int j = - 1 - findEntry(e.getKey());
        bucket[j] = e;
      }
    }
  }
  /** Removes the key-value pair with a specified key. */
  public V remove (K key) throws InvalidKeyException {
    int i = findEntry(key);     // find this key first
    if (i < 0) return null;     // nothing to remove
    V toReturn = bucket[i].getValue();
    bucket[i] = AVAILABLE;              // mark this slot as deactivated
    n--;
    return toReturn;
  }
}