How to convert Map<String, List<String>> to Set<String> using streams?

Question

I'm trying to convert Map> to a List/Set of Strings in single line but couldn't do it unless I use traditional forEach for map. Is anyone aware to do it in single line using streams.

import java.util.*;

public class StreamDemo {
    public static void main(String[] args) {
        HashMap<String, List<String>> map = new HashMap<>();
        ArrayList<String> s1 = new ArrayList<>();
        map.put("A", Arrays.asList("A1, A2"));
        map.put("B", Arrays.asList("B1, B2"));
        map.put("C", Arrays.asList("C1, C2"));

        Set<String> stringValues = new HashSet<>();

        // LOGIC TO GET ALL VALUES A1, A2, B1, B2, C1, C2 INTO stringValues

        System.out.println(stringValues);
        // Expected output
        // [A1, A2, B1, B2, C1, C2]

    }
}

Yassin Hajaj · Accepted Answer · 2020-03-03 14:13:33Z

6

you could use the following stream which basically streams over your values and collect their items inside of a HashSet but note that the HashSet doesn't guarantee sorting. For that you'd want to use TreeSet::new in the end

Set<String> stringValues = map.values()
   .stream()
   .flatMap(List::stream)
   .collect(Collectors.toCollection(HashSet::new));

edited Mar 3, 2020 at 14:13

answered Mar 3, 2020 at 14:12

Yassin Hajaj

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3 Comments

VGR Over a year ago

Collectors.toSet() is more succinct.

Arvind Kumar Avinash Over a year ago

Instead of Collectors.toCollection(HashSet::new) you could use Collectors.toSet().

Yassin Hajaj Over a year ago

@ArvindKumarAvinash Depends on whether you want to enforce the implementation. toSet() doesn't guarantee anything

David Pérez Cabrera · Accepted Answer · 2020-03-03 14:26:44Z

4

A solution without using streams can be even simpler:

map.values().forEach(l -> l.forEach(stringValues::add));

answered Mar 3, 2020 at 14:26

David Pérez Cabrera

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Comments

Prog_G · Accepted Answer · 2020-03-03 14:22:05Z

1

Try the below code:

HashMap<String, List<String>> map = new HashMap<>();
        ArrayList<String> s1 = new ArrayList<>();
        map.put("A", Arrays.asList("A1, A2"));
        map.put("B", Arrays.asList("B1, B2"));
        map.put("C", Arrays.asList("C1, C2"));
        HashSet<String> strings = new LinkedHashSet<>();
        map.values().stream().sequential().forEach(value -> {
            value.forEach(value1 -> {
                strings.add(value1);
            });
        });
        System.out.println(strings);

answered Mar 3, 2020 at 14:22

Prog_G

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Comments

Arvind Kumar Avinash · Accepted Answer · 2020-03-03 14:48:24Z

Since your expected output is [A1, A2, B1, B2, C1, C2], use LinkedHashSet instead of a HashSet.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;

public class Main {
    public static void main(String[] args) throws InterruptedException {
        HashMap<String, List<String>> map = new HashMap<>();
        ArrayList<String> s1 = new ArrayList<>();
        map.put("A", Arrays.asList("A1, A2"));
        map.put("B", Arrays.asList("B1, B2"));
        map.put("C", Arrays.asList("C1, C2"));
        Set<String> stringValues = new LinkedHashSet<>();
        map.values().forEach(list -> list.forEach(stringValues::add));
        System.out.println(stringValues);
    }
}

Output:

[A1, A2, B1, B2, C1, C2]

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