Java集 <String> 在HashMap中

Question

I have this object:

Map<Long, Set<String>> resourcesByID = new HashMap<Long, Set<String>>();

and another:

Set<String> resources;

When I want to put more Set into the HashMap it will override the elements of the Set, that I put into previously. Why? And how can I resolve this problem?

Answer 1

Assuming

Long id = 1234L;
Set<String> resources = ...;

If you want to add another mapping from a Long to a Set then

resourcesById.put(id, resources);

If you want to add String s to an existing mapping

resourcesById.get(id).addAll(resources);

And to make life simpler if you're not doing initialization correctly...

if (!resourcesById.containsKey(id)) {
    resourcesById.put(id, new HashSet<String>());
}
resourcesById.get(id).addAll(resources);

Answer 2

• First reason you may be using same key for each set.
• Second reason if key is different than for each key (long) you might be using same Set using resources reference, you have to create the separate instance of Set for each key.

Answer 3

If key matches, it overrides the previous values, otherwise it won't override.

(or)

As Thomas commented, if your set is mutate and you are updating the same set, yes, then it may override.