CODEWITHSUNDEEP
javahashmapconcurrenthashmap
gautam
gautam

Reputation: 197

Removal of subset of values from a hashmap in java

Hi I've a Map variable defined as follow:

Map<String,List<Event>> myMap = new HashMap<String,List<Event>>();

Lets say the (key:value) pairs are (abc:(x1,x2,x3)), (pqr:(x2,x4,x5)) and (xyz:(x4,x2,x9)).

Now, to remove we would be doing myMap.remove("pqr"), but that would remove the whole (key:value) pair set. But what I want is to just remove x5 in the set(x2,x4,x5) so that it leads to (pqr:(x2,x4)).

Please help me out in achieving this by suggesting some efficient ways to do so.

Upvotes: 1

Views: 673

Answers (5)

Kostas Kryptos
Kostas Kryptos

Reputation: 4111

I am not aware of your full specs and reqs, but in order to decide on the structure used to store Events, you have to consider if duplicates or/and ordering are a requirement too.

Regarding efficiency, due to the fact that you need to search for an Object (to remove it), I would go with a 

Map<String, Set<Event>> myMap = new HashMap<String, Set<Event>>();

or Multimap from Guava.

If ordering is not of your concern, use HashSet<Event> O(1) on add/remove

If natural ordering is required, use a TreeSet<Event> O(logn) on add/remove

If insertion ordering is required, use a LinkedHashSet<Event> O(1) on add/remove

Note: If you accept duplicates (which would be a good reason for choosing a List over a Set) the myMap.get(pqr).remove(x5); would only remove the first occurence of this object. If you need to remove every occurence of x5, then the following will work:

myMap.get(pqr).removeAll(Collections.singleton(x5));

Upvotes: 0

javapapo
javapapo

Reputation: 1342

I don't know if you can really do this (meaning config restrictions) but it seems that you have some kind of MultiMap requirements.

I would suggest (if you can) to take a look on some MultiMap implementations, for example Guava. See related here or here

Upvotes: 0

Manoj Suthar
Manoj Suthar

Reputation: 1455

Considering the values to be ArrayList, you'll have to get the value of the key "pqr" and remove the data from the arraylist as answered above.

Upvotes: 1

Jean Logeart
Jean Logeart

Reputation: 53829

Simply:

myMap.get(pqr).remove(x5);

Upvotes: 3

TangledUpInBlue
TangledUpInBlue

Reputation: 746

myMap.put(key, value) will update the key, value pairing (and return the original value), so I would suggest just putting the new values you want in the map.

Upvotes: 0

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