Merge maps from a list of maps java8

Question

Hello fellow developers, Need your expertise on below problem I have a below incoming list of maps

Map<String, Object> m1 = new HashMap<String, Object>();
m1.put("name", "alex");
m1.put("age", "40");
l1.add(m1);
m1 = new HashMap<String, Object>();
m1.put("name", "alex");
m1.put("state", "Texas");
l1.add(m1);

m1 = new HashMap<String, Object>();
m1.put("name", "alice");
m1.put("age", "35");
l1.add(m1);
m1 = new HashMap<String, Object>();
m1.put("name", "alice");
m1.put("state", "Arizona");
l1.add(m1);

m1 = new HashMap<String, Object>();
m1.put("name", "bob");
m1.put("age", "25");
l1.add(m1);
m1 = new HashMap<String, Object>();
m1.put("name", "bob");
m1.put("state", "Utah");
l1.add(m1);

I want the output list of map as below using java 8 streams-

[{name="alex", age="40", state="Texas"}
{name="alice", age="35", state="Arizona"}
{name="bob", age="25", state="Utah"}]

Appreciate any help here.

What did you try?

Naman
– Naman

2020-03-16 01:23:25 +00:00
Commented Mar 16, 2020 at 1:23 — Naman
– Naman, Commented Mar 16, 2020 at 1:23

Hadi · Accepted Answer · 2020-03-15 20:36:37Z

2

You can use toMap() collector with merge function.

l1.stream()
   .collect(Collectors.toMap(m -> m.get("name").toString(), Function.identity(),
               (map1, map2) -> { map1.putAll(map2); return map1;})
           )
   .values();

Indeed in this solution, we merge maps based on name key value. so if two maps have the same value for name key then they will merge. since toMap() collector's result is Map<String,Map<String,Object>> here and desire result is Map<String,Object> so we have called values()

edited Mar 15, 2020 at 20:36

answered Mar 15, 2020 at 20:17

Hadi

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