guava/src/com/google/common/collect/ForwardingSortedMap.java
@Beta
  protected SortedMap<K, V> standardSubMap(K fromKey, K toKey) {
    checkArgument(unsafeCompare(fromKey, toKey) <= 0, "fromKey must be <= toKey");
    return tailMap(fromKey).headMap(toKey);
  }
@Beta
  protected SortedMap<K, V> standardSubMap(K fromKey, K toKey) {
    checkArgument(unsafeCompare(fromKey, toKey) <= 0, "fromKey must be <= toKey");
    return tailMap(fromKey).headMap(toKey);
  }
A sensible default implementation of {@link #subMap(Object, Object)} in terms of {@link
#headMap(Object)} and {@link #tailMap(Object)}. In some situations, you may wish to override
{@link #subMap(Object, Object)} to forward to this implementation.

@since 7.0