code: @Override
      public Set<K> keySet() {
          try {
              lockAllSegments();
              Set<K> set = new HashSet<>();
              for (LinkedHashMapSegment<K, V> seg : segments) {
                  set.addAll(seg.keySet());
              }
              return set;
          } finally {
              unlockAllSegments();
          }
      }
docs: Gets the all key in this map.
  
  @return a set view of the keys contained in this map