Processing a list of Map<String,List<Object>> in java 8

Question

I am trying to create a single map from list of maps. Which contains only key "1" and all the values of key "1" across different maps under that list using Java 8 stream API.

List<Map<String,Object>> list=new ArrayList<>();
Map<String,Object> map1=new HashMap<>();
map1.put("1", Arrays.asList(new String[] {"A"}));
map1.put("2", Arrays.asList(new String[] {"B"}));

Map<String,Object> map2=new HashMap<>();
map2.put("1", Arrays.asList(new String[] {"C"}));
map2.put("2", Arrays.asList(new String[] {"D"}));

Required output :- {1=[A, C]}

Why don't you declare list as List<Map<String,List<Object>>>? Otherwise why isn't {1=[[A], [C]]} the expected output? — ernest_k
– ernest_k, Commented Jan 24, 2020 at 7:38
@ernest_k I can Change the declaration to List<Map<String,List<Object>>> but I require output as {1=[A, C]}. {1=[A, C]} will be list of list right ? — crr
– crr, Commented Jan 24, 2020 at 7:52
@Chirag Right, that is exactly what is unclear with your question right now. Given the current code and List<Map<String,List<Strinig>>> as input type, the very first question you should ask yourself is why do you need such a complex representation of data, is there no way tto simplify it? — Naman
– Naman, Commented Jan 24, 2020 at 8:10
@Naman Need to return two lists from a process. So I am maintaining a map for it and that process can return multiple map, which makes it List of map. Any better suggestion — crr
– crr, Commented Jan 24, 2020 at 9:02

Youcef LAIDANI · Accepted Answer · 2020-01-24 08:21:02Z

5

Because you have only one entry, then in this case, you need just to focus on the values, and for the key you can just use "1", for that you can create a Map like this :

Map<String, List<String>> result = new HashMap<>();
result.put("1", list.stream()
        .filter(e -> e.containsKey("1"))
        .flatMap(e -> e.values().stream())
        .flatMap(List::stream)
        .collect(Collectors.toList()));

Or as stated by Lino in the comment, you can also use :

Map<String, List<String>> result = list.stream()
    .filter(e -> e.containsKey("1"))
    .flatMap(e -> e.values().stream())
    .flatMap(List::stream)
    .collect(Collectors.groupingBy(t -> "1"));

edited Jan 24, 2020 at 8:21

answered Jan 24, 2020 at 7:54

Youcef LAIDANI

60.3k21 gold badges111 silver badges178 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Lino Over a year ago

if you don't want to put into a map directly, you could use a groupingBy collector instead of toList: .collect(Collectors.groupingBy(i -> "1", i -> i))

Youcef LAIDANI Over a year ago

@LinosaysReinstateMonica I don't get you sorry

Lino Over a year ago

Sorry I meant groupingBy(i -> "1") as can be seen here. Also i noticed that your code doesn't compile. Your stream collects into a List<List<String>> I think you missed a flatMap

Youcef LAIDANI Over a year ago

Thank you @LinosaysReinstateMonica good idea, I put it in my answer :)

Lino · Accepted Answer · 2020-01-24 08:17:24Z

4

As mentioned by @ernest_k you should declare list as: List<Map<String, List<String>>>

You can use a groupingBy collector:

Map<String, List<String>> result = list.stream()
    .map(m -> m.get("1"))
    .filter(Objects::nonNull)
    .flatMap(Collection::stream)
    .collect(Collectors.groupingBy(t -> "1"));

You can get rid of the intermediate filter with this. But this may be more confusing (and effectively does the same):

Map<String, List<String>> result = list.stream()
    .map(m -> m.get("1"))
    .flatMap(l -> l == null ? Stream.empty() : l.stream())
    .collect(Collectors.groupingBy(t -> "1"));

answered Jan 24, 2020 at 8:17

Lino

20k6 gold badges55 silver badges73 bronze badges

2 Comments

Ravindra Ranwala Over a year ago

Why don't you use this

Map<String, List<String>> result = list.stream().filter(m -> m.containsKey("1")) 				.flatMap(m -> m.get("1").stream()).collect(Collectors.groupingBy(l -> "1"));

Lino Over a year ago

@RavindraRanwala that works as well, but does also effectively the same doesn't it? The map implementations I know almost always check if get(key) != null for containsKey

Hadi · Accepted Answer · 2020-01-24 08:35:37Z

4

You can do:

Map<String, List<Object>> result = list.stream()
            .filter(entry -> entry.containsKey("1"))
            .map(entry -> (List<Object>) entry.get("1"))
            .collect(Collectors.toMap(t1 -> "1", 
                          ArrayList::new, (l1, l2) -> { l1.addAll(l2);return l1; }));

or use groupingBy collector :

Map<String, List<Object>> result2 = list.stream()
            .filter(entry -> entry.containsKey("1"))
            .flatMap(entry -> ((List<Object>) entry.get("1")).stream())
            .collect(Collectors.groupingBy(t->"1",Collectors.
                        collectingAndThen(Collectors.toList(),ArrayList::new)));

edited Jan 24, 2020 at 8:35

answered Jan 24, 2020 at 7:58

Hadi

17.3k4 gold badges41 silver badges63 bronze badges

1 Comment

Lino Over a year ago

Why not use groupingBy?

Collectives™ on Stack Overflow

Processing a list of Map<String,List<Object>> in java 8

3 Answers 3

4 Comments

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related