Get only key from List of Map object using stream

Question

I am trying to get the Get only the key values from the List of Map object using the stream in java 8.

When I stream the List of map object I am getting Stream<List<String>> instead of List<String>.

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class StreamTest {

public static void main(String[] args) {
    // TODO Auto-generated method stub
    System.out.println("Hello World");
    Map<String, String> a = new HashMap<String, String>();
    a.put("1", "Bharathi");
    a.put("2", "Test");
    a.put("3", "Hello");
    List<Map<String, String>> b = new ArrayList<>();
    b.add(a);
    System.out.println("Hello World" + b);

    /*
     * b.stream().map(c-> c.entrySet().stream().collect( Collectors.toMap(entry ->
     * entry.getKey(), entry -> entry.getValue())));
     */

    Stream<List<String>> map2 = b.stream()
            .map(c -> c.entrySet().stream().map(map -> map.getKey()).collect(Collectors.toList()));
    //List<List<String>> collect = map2.map(v -> v).collect(Collectors.toList());

  }

}

How to get the key object from the List of Map object?

Naman · Accepted Answer · 2020-01-21 06:32:57Z

6

you can use flatMap over the keySet of each Map within:

List<String> output = lst.stream()
            .flatMap(mp -> mp.keySet().stream())
            .collect(Collectors.toList());

answered Jan 21, 2020 at 6:32

Naman

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1 Comment

Naman Over a year ago

Aside: There is not much benefit that you get from modeling the objects as List<Map<String, String>> instead of a custom class and then further using a List<CustomClass>.

Andronicus · Accepted Answer · 2020-01-21 06:32:29Z

1

You can simply flatMap it:

b.stream().flatMap(m -> m.keySet().stream()).collect(Collectors.toList())

answered Jan 21, 2020 at 6:32

Andronicus

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Comments

Hadi · Accepted Answer · 2020-01-21 06:53:45Z

1

Of course flatMap is stream base solution! however you can do it with non-stream version in simple way.

List<String> map2 = new ArrayList<>();
b.forEach(map -> map2.addAll(map.keySet()));

edited Jan 21, 2020 at 6:53

answered Jan 21, 2020 at 6:40

Hadi

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Comments

dkovalenko · Accepted Answer · 2020-01-21 07:00:09Z

0

You also can use a slightly more declarative way:

List<String> collect = b.stream()
                .map(Map::keySet) // map maps to key sets
                .flatMap(Collection::stream) // union all key sets to the one stream
                .collect(Collectors.toList()); // collect the stream to a new list

answered Jan 21, 2020 at 7:00

dkovalenko

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