@Override
  public String getLabel(int ordinal) {
    this.lock.readLock().lock();
    try {
      return this.ordinalVsLabels.get(ordinal);
    } finally {
      this.lock.readLock().unlock();
    }
  }
@param ordinal The ordinal of label which we are looking for.
@return The label having the given ordinal. Returns <code>null</code> when no label exist in
        the system with given ordinal
hbase-server/src/main/java/org/apache/hadoop/hbase/security/visibility/VisibilityLabelsCache.java