How merge list when combine two hashMap objects in Java

Question

I have two HashMaps defined like so:

HashMap<String, List<Incident>> map1 = new HashMap<String, List<Incident>>();
HashMap<String, List<Incident>> map2 = new HashMap<String, List<Incident>>();

Also, I have a 3rd HashMap Object:

HashMap<String, List<Incident>> map3;

and the merge list when combine both.

Look at here http://stackoverflow.com/questions/4299728/how-can-i-combine-two-hashmap-objects-of-same-type-in-java — Jayram, Jul 12 '13 at 05:26
This is quite a different question than the "How can I combine two HashMap objects containing the same types?" This question is about combining **Multi-value** maps. The problem is that a solution for combining the values in the `List` is needed. map.putAll() will replace the list, not combine the two. — Zombies, May 09 '15 at 03:56

Chase · Accepted Answer · 2013-07-12T05:30:36.477

7

In short, you can't. map3 doesn't have the correct types to merge map1 and map2 into it.

However if it was also a HashMap<String, List<Incident>>. You could use the putAll method.

map3 = new HashMap<String, List<Incident>>();
map3.putAll(map1);
map3.putAll(map2);

If you wanted to merge the lists inside the HashMap. You could instead do this.

map3 = new HashMap<String, List<Incident>>();
map3.putAll(map1);
for(String key : map2.keySet()) {
    List<Incident> list2 = map2.get(key);
    List<Incident> list3 = map3.get(key);
    if(list3 != null) {
        list3.addAll(list2);
    } else {
        map3.put(key,list2);
    }
}

edited Jul 12 '13 at 05:30

answered Jul 12 '13 at 05:19

Chase

1,419
12
17

4

Maybe OP want to merge `"a"->[1,2]` and `"a"->[3,4]` into `"a"->[1,2,3,4]`, since the value of the map is a `List`. – johnchen902 Jul 12 '13 at 05:24
Good, point, i'll add something on that. – Chase Jul 12 '13 at 05:26
yes, i want to merge list – martinixs Jul 12 '13 at 05:27

score 4 · Answer 2 · edited May 23 '17 at 12:16

create third map and use putAll() method to add data from ma

HashMap<String, Integer> map1 = new HashMap<String, Integer>();

HashMap<String, Integer> map2 = new HashMap<String, Integer>();

HashMap<String, Integer> map3 = new HashMap<String, Integer>();
map3.putAll(map1);
map3.putAll(map2);

You have different type in question for map3 if that is not by mistake then you need to iterate through both map using EntrySet

score 0 · Answer 3 · edited Dec 28 '18 at 19:10

0

Use commons collections:

Map<String, List<Incident>> combined = CollectionUtils.union(map1, map2);

If you want an Integer map, I suppose you could apply the .hashCode method to all values in your Map.

edited Dec 28 '18 at 19:10

Paul

19,704
14
78
96

answered Jul 12 '13 at 05:34

hd1

33,938
5
80
91

`CollectionUtils.union` doesn't work with maps, only collections. – Paul Dec 28 '18 at 19:11

score -1 · Answer 4 · answered Jul 12 '13 at 05:31

-1

HashMap has a putAll method.

Refer this : http://docs.oracle.com/javase/6/docs/api/java/util/HashMap.html

answered Jul 12 '13 at 05:31

Manish Doshi

1,205
1
9
17

How merge list when combine two hashMap objects in Java

4 Answers4

Linked