code: public Collection keyCollection() {
          if (keyCollection == null) {
              keyCollection = new AbstractCollectionView() {
                  public long size() throws ObjectManagerException {
                      return AbstractTreeMap.this.size();
                  }
  
                  public Iterator iterator() throws ObjectManagerException {
                      return makeTreeMapIterator(
                      new AbstractMapEntry.Type() {
                          public Object get(AbstractMapEntry entry) {
                              return entry.key;
                          }
                      });
                  }
              };
          }
          return keyCollection;
      }
docs: Answers a Collection of the keys contained in this TreeMap. The set is backed by
  this TreeMap so changes to one are relected by the other. The Collection does
  not support adding.
  
  @return a Collection of the keys