CODEWITHSUNDEEP
javadata-structuresjava-8hashmapjava-stream
User_Targaryen
User_Targaryen

Reputation: 4225

Create a Map out of a List of String(s) with streams

I have a List of String(s), but I want to convert it into a Map<String, Boolean> from the List<String>, making all of the boolean mappings set to true. I have the following code.

import java.lang.*;
import java.util.*;
class Main {
  public static void main(String[] args) {
    List<String> list = new ArrayList<>();
    list.add("ab");
    list.add("bc");
    list.add("cd");
    Map<String, Boolean> alphaToBoolMap = new HashMap<>();
    for (String item: list) {
      alphaToBoolMap.put(item, true);
    }
    //System.out.println(list); [ab, bc, cd]
    //System.out.println(alphaToBoolMap);  {ab=true, bc=true, cd=true}
  }
}

Is there a way to reduce this using streams?

Upvotes: 8

Views: 7106

Answers (3)

Misha
Misha

Reputation: 28133

If you are going to mutate an existing Map<..., Boolean>, you can use Collections.newSetFromMap. It lets you treat such a map as a Set: 

Collections.newSetFromMap(alphaToBoolMap).addAll(list);

Upvotes: 0

fps
fps

Reputation: 34450

The shortest way I can think of is not a one-liner, but it's indeed short:

Map<String, Boolean> map = new HashMap<>();
list.forEach(k -> map.put(k, true));

This is personal taste, but I only use streams when I need to apply transformations to the source, or filter out some elements, etc.

As suggested in the comments by @holi-java, many times, using a Map with Boolean values is pointless, since there are only two possible values to map keys to. Instead, a Set could be used to solve almost all the same problems you'd solve with a Map<T, Boolean>.

Upvotes: 8

Elliott Frisch
Elliott Frisch

Reputation: 201409

Yes. You can also use Arrays.asList(T...) to create your List. Then use a Stream to collect this with Boolean.TRUE like

List<String> list = Arrays.asList("ab", "bc", "cd");
Map<String, Boolean> alphaToBoolMap = list.stream()
        .collect(Collectors.toMap(Function.identity(), (a) -> Boolean.TRUE));
System.out.println(alphaToBoolMap);

Outputs

{cd=true, bc=true, ab=true}

For the sake of completeness, we should also consider an example where some values should be false. Maybe an empty key like

List<String> list = Arrays.asList("ab", "bc", "cd", "");

Map<String, Boolean> alphaToBoolMap = list.stream().collect(Collectors //
        .toMap(Function.identity(), (a) -> {
            return !(a == null || a.isEmpty());
        }));
System.out.println(alphaToBoolMap);

Which outputs

{=false, cd=true, bc=true, ab=true}

Upvotes: 11

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