CODEWITHSUNDEEP
javajava-streamcollectors
Ashish Mishra
Ashish Mishra

Reputation: 155

Java 8 List from String and List of String

I want to generate a list of strings, comprising id and external ids, from a list of Bean.

public class User {
    private String id;
    private List<String> externalIds;
}

I got it using the below code, but here I need to do the stream twice.

List<User> references = new ArrayList();
Stream.concat(references.stream().map(User::getId),
references.stream().map(User::getExternalIds).flatMap(Collection::stream))
            .collect(Collectors.toList());

Is there any better way to rewrite this code?

Upvotes: 1

Views: 3922

Answers (3)

WJS
WJS

Reputation: 40057

If you're using Java 16+ you can do it using mapMulti. This essentially intercepts each item in the stream and replaces it with other items. It avoids re-streaming and flatmapping. Since the nature of the stream(i.e. type) is being changed from User to String, the witness before mapMulti is required.

  • add the user id in the stream.
  • then add each external id in the stream for that user
List<String> results = references.stream()
        .<String>mapMulti((user, consumer) -> {
            consumer.accept(user.getId());
            for (String id : user.getExternalIds()) {
                consumer.accept(id);
            }
        }).toList();

Upvotes: 0

Bohemian
Bohemian

Reputation: 425398

Try this:

references.stream()
  .flatMap(u -> Stream.of(List.of(u.getId()), u.getExternalIds()))
  .flatMap(List::stream)
  .collect(Collectors.toList());

Upvotes: 1

Rob Spoor
Rob Spoor

Reputation: 9175

Use Stream.concat inside the flatMap operation:

references.stream()
        .flatMap(user -> Stream.concat(
            Stream.of(user.getId()),
            user.getExternalIds().stream()
        ))
        .collect(Collectors.toList())

Upvotes: 8

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