Question

I have a SortedMap, ordered according to the natural ordering of its keys. Can I safely cast its keySet() to a SortedSet, without risking an invalid cast exception.

That is, will the following throw?

SortedMap<K, V> map = ...;
SortedSet<K> set = (SortedSet<K>) map.keySet();

If the answer is “depends on the implementation of SortedMap”, is this at a minimum safe for a TreeMap?

Answer 1

For SortedMap in general, no as it is not documented in the javadoc.

However, TreeMap also implements NavigableMap, which uses a NavigableSet as a key set and NavigableSet extends SortedSet…

So what you can do is:

NavigableMap<K, V> map = ...; //can be a TreeMap or a ConcurrentSkipListMap
SortedSet<K> set = map.navigableKeySet();