Java stream, get multiple properties from object

Question

I have an object like this:

public class Transaction {
    private long buyerId;
    private long sellerId;
... }

Given a List<Transaction>, I want the ids of all the buyers/sellers involved in those transactions, e.g. in a Collection<Long>. This is my working solution:

List<Transaction> transactions = getTransactions();
Stream<Long> buyerIds = transactions.stream().map( Transaction::getBuyerId );
Stream<Long> sellerIds = transactions.stream().map( Transaction::getSellerId );
Collection<Long> allBuyerSellerIds = Stream.concat( buyerIds, sellerIds )
                .collect( Collectors.toList() );

However, it would be nice if I could do this without iterating through transactions twice. How can I do this? i.e. I want to iterate through transactions with a stream, and get multiple properties each time.

Thanks.

Are you sure you want toList() rather than toSet()? A List may have duplicates, while a Set will not. — VGR
– VGR, Commented Mar 3, 2021 at 19:26

daniu · Accepted Answer · 2021-03-03 12:53:36Z

4

You can use flatMap since you want them in one collection in the end anyway.

getTransactions().stream()
 .flatMap(t -> Stream.of(t.getBuyerId(), t.getSellerId()))
 .collect(toList());

If you didn't want them in a single collection, you might be better off with a conventional loop instead.

answered Mar 3, 2021 at 12:53

daniu

15.1k7 gold badges37 silver badges63 bronze badges

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

Collectives™ on Stack Overflow

Java stream, get multiple properties from object

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related