Intersection of java.util.Map

Question

Is there a method in java.util.Map or any util to perform an intersection on two maps? (To intersect two maps by the "keys")

I am not able to find any. I can always implement my own intersection logic, but I was hoping there is already some operation in one of the java.util.* classes that would do this.

Do you want to see if the keys of one map are also keys in the other map? Do the values also matter, or just the keys? — dashrb, Nov 01 '12 at 15:40
@mandy, please update your question with this information. :-) — Pikachu, Aug 19 '16 at 16:10

Martin Ellis · Accepted Answer · 2012-11-01T16:18:30.987

60

How about:

Map map1 = ...;
Map map2 = ...;
Map result = new ...(map1);
result.keySet().retainAll(map2.keySet());

or:

Map map1 = ...;
Map map2 = ...;
Set result = new ...(map1.keySet());
result.retainAll(map2.keySet());

edited Nov 01 '12 at 16:18

answered Nov 01 '12 at 15:43

Martin Ellis

9,603
42
53

I have 2 maps, map1 and map2. I want to perform an intersection between map1 and map2. I would have to create a new Set if i try to use this approach. – mandy Nov 01 '12 at 15:45
That is an optional operation, will it work for all such keySets? – Woot4Moo Nov 01 '12 at 15:49
By 'that' you mean Set#retainAll(). It'll work with java.util.* collections, and the concurrent collections. I'd expect it to work with all popular implementations of mutable maps, but obviously you'd need to verify for the particular implementation. @ruakh: Good edit, thanks. – Martin Ellis Nov 01 '12 at 15:59
So now you have a `Map` with the values from `map1` for intersecting keys. What about such values from `map2`? – darrengorman Nov 01 '12 at 16:03
See the clarification in the comments on the question: the keys are of interest, the values "do not matter". – Martin Ellis Nov 01 '12 at 16:04
I saw that, so why bother creating a new `result` Map at all? One of the implementations from Apache Commons or Guava will get the intersecting keys if that's all @mandy is interested in. – darrengorman Nov 01 '12 at 16:12
1

Fair point, I've updated the answer to include an option for just obtaining a set. I suggested a JRE method because it seems likely to be more applicable. That is: not everyone can easily add dependencies to their application (for example, because binary size is a problem, bureaucracy, etc), nor necessarily wants to for something that they can easily do without. – Martin Ellis Nov 01 '12 at 16:21
I found second answer very useful. Not needing externals like Apache Commons or Guava is a real bonus (to me). I find an answer that is two lines need not be in the JRE. Perhaps this could be placed in your own Utils function and hardened against nulls and Exceptions, and the Set could/should be typed. I see no reason to include any of that in the answer as this answers the question concisely. Well done. – brightmatter Mar 16 '21 at 16:31

score 16 · Answer 2 · edited Feb 08 '18 at 11:43

16

If you're using Guava, you can use Maps.difference to get a MapDifference object, from which you can extract the entriesInCommon() and entriesDiffering() as maps. (Disclosure: I contribute to Guava.)

edited Feb 08 '18 at 11:43

Iulian Popescu

2,595
4
23
31

answered Nov 01 '12 at 16:07

Louis Wasserman

191,574
25
345
413

The OP seems to be only be concerned about the keys. The Guava library is not clearly documented in this respect, but it seems to be comparing also the values, no? Please document appropriately. – Roland Aug 30 '16 at 14:28
Read the docs for `MapDifference` -- it tells you that `entriesDiffering()`, for example, "returns an unmodifiable map describing keys that appear in both maps, but with different values." – Louis Wasserman Aug 30 '16 at 19:09

score 4 · Answer 3 · answered Nov 01 '12 at 15:43

4

Guava's Sets.intersection(Set, Set) should do the job, with the keySet of each Map passed in as parameters.

answered Nov 01 '12 at 15:43

darrengorman

12,952
2
23
24

How would you convert that back into a `Map`? – ruakh Nov 01 '12 at 15:45
Well that depends on what you want to do with the values corresponding to intersecting keys surely? – darrengorman Nov 01 '12 at 15:49
The OP appears to only care about the keys, so it's fine. – Louis Wasserman Nov 01 '12 at 16:34

calderonmluis · Answer 4 · 2012-11-01T15:54:25.663

0

To test for intersection you can use the containsAll() operation. Which 'Returns true if this set contains all of the elements of the specified collection. If the specified collection is also a set, this method returns true if it is a subset of this set.'

To get a collection of these intersecting elements you can use the retainAll() operation instead.

These methods are both found here

http://docs.oracle.com/javase/1.4.2/docs/api/java/util/Set.html

edited Nov 01 '12 at 15:54

answered Nov 01 '12 at 15:43

calderonmluis

547
4
11

That doesn't give you the intersection, though. It just tells you whether one set of keys is a subset of the other. Finding the intersection means returning the common keys/values. – Martin Ellis Nov 01 '12 at 15:50

score 0 · Answer 5 · answered Nov 01 '12 at 15:46

0

I would recommend apache collectionUtils#intersection

Do the following:

 Collection intersection=    
      CollectionUtils.intersection(map1.keySet(),map2.keySet());

answered Nov 01 '12 at 15:46

Woot4Moo

23,987
16
94
151

dashrb · Answer 6 · 2019-07-25T21:10:13.847

0

Loop over one map's keys, see if they're in the second map:

private Map getIntersection(Map mapOne, Map mapTwo)
{
    Map intersection = new HashMap();
    for (Object key: mapOne.keySet())
    {
        if (mapTwo.containsKey(key))
           intersection.put(key, mapOne.get(key));
    }
    return intersection;
}

edited Jul 25 '19 at 21:10

answered Nov 01 '12 at 15:49

dashrb

952
4
10

Set intersection = new HashMap(); ? Really? – Partha Jul 24 '19 at 20:31
hahaha oops. That should say "Map". I'll edit. (Thanks for noticing, after 7 years. I can only assume my real code has similar latent but obvious bugs!) – dashrb Jul 25 '19 at 21:09
Your code helped me, and that's why I cared to correct it :) Thanks! – Partha Jul 26 '19 at 18:28

Intersection of java.util.Map

6 Answers6

Linked

Related