Question

When inspecting Map and SortedMap interfaces, I have noticed many methods that are already defined within Map interface are also redefined (not simply let inherited) within the SortedMap.

SortedMap extending Map, why redeclaring some methods like for instance:

Set<K> keySet();
Collection<V> values();

Why this redundancy?

(I use JDK 7)

Answer 1

I would say that methods are declared in interfaces and methods are defined in classes.

Map are not guaranteed to be ordered but Sorted Maps are. This means what each method will guarantee is different and needs different documentation.

The Javadoc for Map.keySet() is highlighting the differences

Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator’s own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Returns:
a set view of the keys contained in this map

The Javadoc for SortedMap.keySet() is

Returns a Set view of the keys contained in this map. The set’s iterator returns the keys in ascending order. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator’s own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Specified by:
keySet in interface Map

Returns:
a set view of the keys contained in this map, sorted in ascending order