Method subMap

guava/src/com/google/common/collect/Maps.java:981–984

(@ParametricNullness K fromKey, @ParametricNullness K toKey)

    }

    @Override
    public SortedMap<K, V> subMap(@ParametricNullness K fromKey, @ParametricNullness K toKey) {
      return asMap(backingSet().subSet(fromKey, toKey), function);
    }

    @Override
    public SortedMap<K, V> headMap(@ParametricNullness K toKey) {