# retainAll method

void retainAll (Iterable<Object> elements)
override

Removes all elements of this set that are not elements in `elements`.

Checks for each element of `elements` whether there is an element in this set that is equal to it (according to `this.contains`), and if so, the equal element in this set is retained, and elements that are not equal to any element in `elements` are removed.

## Implementation

    void retainAll(Iterable<Object> elements) {
      // Build a set with the same sense of equality as this set.
      SplayTreeSet<E> retainSet = SplayTreeSet<E>(_comparator, _validKey);
      int modificationCount = _modificationCount;
      for (Object object in elements) {
        if (modificationCount != _modificationCount) {
          // The iterator should not have side effects.
          throw ConcurrentModificationError(this);
        }
        // Equivalent to this.contains(object).
        if (_validKey(object) && _splay(object) == 0) {
          retainSet.add(_root.key);
        }
      }
      // Take over the elements from the retained set, if it differs.
      if (retainSet._count != _count) {
        _root = retainSet._root;
        _count = retainSet._count;
        _modificationCount++;
      }
    }