From Map<String, Set<Integer>> collect values from a Set<String> of keys

Question

I am trying to use map to map a set of keys into a Map of String to Set of Integer. Ideally I want to get all the value sets and collect them into a single set.

Lets say I have:

Map<String, List<Integer>> keyValueMap = new HashMap<>();
Set<String> keys = new HashSet<>();
Set<String> result = new HashSet<>();

I have tried:

result.addAll(keys.stream().map(key -> keyValueMap.get(key)).collect(Collectors.toSet());

This nets me an error saying addAll() is not applicable for the type Set>. I have tried replacing map() with flatMap() but I can't seem to get the syntax right if that is the solution.

What is the correct syntax to make this work?

Thanks!

Jacob G. · Accepted Answer · 2020-06-05 19:25:01Z

2

It looks like the type of result should be Set<Integer> instead of Set<String>.

With your snippet, you're attempting to invoke Set#addAll on a Set<Integer>, but the argument being passed is a Set<List<Integer>>, which doesn't compile.

To ameliorate your issue, one solution is to use flatMap instead of map:

result.addAll(keys.stream()
                  .flatMap(key -> keyValueMap.get(key).stream())
                  .collect(Collectors.toSet()));

A logically equivalent snippet is:

result.addAll(keys.stream()
                  .map(keyValueMap::get)
                  .flatMap(List::stream)
                  .collect(Collectors.toSet()));

Another solution would be to utilize Map#values:

result.addAll(keyValueMap.values().stream()
                         .flatMap(List::stream)
                         .collect(Collectors.toSet()));

edited Jun 5, 2020 at 19:25

answered Jun 5, 2020 at 19:08

Jacob G.

29.8k7 gold badges70 silver badges122 bronze badges

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

12 Comments

Jacob G. Over a year ago

@TimB. Ameliorate, meaning to make something better. In this case, it refers to fixing your compiler error. And if I were you, I'd take advantage of Map#keySet and Map#values.

Jacob G. Over a year ago

@TimB. Take a look at the documentation for Map. It has many methods that could potentially suit your use-case.

Tim B. Over a year ago

My use case was actually a lot more complicated than my question above, so I am not quite sure there is better way to take advantage of Map#keySet and Map#values, but I will take another read through the docs and see.

Tim B. Over a year ago

Perfect, thank you! The second solution was not compilable for me for some reason so I am just sticking with the first.

Holger Over a year ago

There is no need for the addAll. Just assign the result of the collect operation to result, e.g.

Set<Integer> result = keys.stream() .flatMap(key -> getOrDefault(key, Collections.emptyList()).stream()) .collect(Collectors.toSet());

|

Collectives™ on Stack Overflow

From Map<String, Set<Integer>> collect values from a Set<String> of keys

1 Answer 1

12 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

12 Comments

Your Answer

Sign up or log in

Post as a guest

Related