@Override
  public K ceilingKey(K key) {
    Map.Entry<K, V> entry = ceilingEntry(key);
    if (entry == null) {
      return null;
    }
    return entry.getKey();
  }
A Map that keeps a sorted array in order to provide the concurrent map interface. Keeping a
sorted array means that it's much more cache line friendly, making reads faster than the tree
version. In order to make concurrent reads and writes safe this does a copy on write. There can
only be one concurrent write at a time.
hbase-common/src/main/java/org/apache/hadoop/hbase/types/CopyOnWriteArrayMap.java