Question

I am trying to implement a complicated data structure that looks like

Map<A,Set<B>> map = new HashMap<A,Set<B>>();

but I am not sure how to add content to this data structure. I am trying to do a

map.put(a,b);

but that doesn’t help as it expects a set. The requirement I have needs to implement this data structure. Any thoughts?

Answer 1

You need to add your element within the set contained in the hash map:

if (!map.containsKey(a)) {
    map.put(a, new Set<B>());
}
map.get(a).add(b);