Question

First, I tried to use

Map<Integer, Set<Integer>> map = new HashMap<Integer, HashSet<Integer>>();

, but it did not compile because no casting is available.

Then I tried

Map<Integer, ? extends Set<Integer>> map = new HashMap<Integer, HashSet<Integer>>();

. It seemed to work, but when I tried to call

map.put(i, new HashSet<Integer>());

, it turned out to be a failure due to uncompatibility.

How can I solve the problem?

Answer 1

You can simply do this:

Map<Integer, Set<Integer>> map = new HashMap<Integer, Set<Integer>>();
map.put(1, new HashSet<Integer>());

When you retrieve the value from the map, then it will be of type Set<Integer>, but that shouldn’t make any difference as per your expectation. But if you want to retrieve only HashSet<Integer> then your map definition should be

Map<Integer, HashSet<Integer>> map = new HashMap<Integer, HashSet<Integer>>();