How to return just one string from java stream

Question

I understand how to collect to a List, but can't figure how I would return just one parameter of filtered object as a String.

fee = new BigDecimal(fees
            .stream()
            .filter(p -> p.getTodate().isAfter(LocalDateTime.now()))
            .filter(p -> p.getFromdate().isBefore(LocalDateTime.now()))
            .filter(p -> p.getId().equals(id))

    return fee;

I first check that the fee is up to date, as there might be upcoming fees and fees that are no longer valid. Then I match the id with remaining fees. But then there is code missing between last filter and return.

I just want to return String from Stream object (p.getFee) for BigDecimal constructor.

I know there is only one Stream object remaining after filters.

If you expect a single element or 0 element in the collected stream you should explicit handle the case as you have more than 1 element from the collect. — davidxxx, Nov 09 '18 at 08:41

score 20 · Accepted Answer · answered Nov 09 '18 at 08:30

20

Use findFirst to return the first element of the Stream that passes your filters. It returns an Optional, so you can use orElse() to set a default value in case the Stream is empty.

fee = new BigDecimal(fees
            .stream()
            .filter(p -> p.getTodate().isAfter(LocalDateTime.now()))
            .filter(p -> p.getFromdate().isBefore(LocalDateTime.now()))
            .filter(p -> p.getId().equals(id))
            .map(p -> p.getFee())
            .findFirst()
            .orElse(/*some default value*/));

answered Nov 09 '18 at 08:30

Eran

374,785
51
663
734

1

Or perhaps throw an exception, if it is an exceptional state. – Magnilex Nov 09 '18 at 09:01
do you happen to know why .filter(p -> p.getInstrumentid().equals(id)) part cant find match even tho i looked it up and data definitely contains fee object with given id? if i take the last filter out, everything else works.. – Clomez Nov 09 '18 at 10:15
@Clomez what's the type of id and the return type of getInstrumentId? – Eran Nov 09 '18 at 11:57

score 4 · Answer 2 · answered Nov 09 '18 at 08:54

4

Maybe it would be a better approach:

fee = fees.stream()
        .filter(p -> p.getTodate().isAfter(LocalDateTime.now()))
        .filter(p -> p.getFromdate().isBefore(LocalDateTime.now()))
        .filter(p -> p.getId().equals(id))
        .map(p -> p.getFee())
        .findFirst()
        .map(BigDecimal::new)
        .orElse(/*some default value*/);

answered Nov 09 '18 at 08:54

Piotr Rzonsowski

41
1

2

You can also change the first `map` operation to `.map(p -> new BigDecimal(p.getFee()))` – Holger Nov 09 '18 at 09:07
but in this case, you could pass null to "new BigDecimal(null)" – Piotr Rzonsowski Nov 09 '18 at 09:11
In which scenario? When `getFee()` returns `null`, your variant will fail with a `NullPointerException` too. – Holger Nov 09 '18 at 09:12
i don't think(but not sure) because if p.getFee()==null then it go orElse – Piotr Rzonsowski Nov 09 '18 at 09:13
5

Recommended read: [Why does findFirst() throw a NullPointerException if the first element it finds is null?](https://stackoverflow.com/q/32466799/2711488) – Holger Nov 09 '18 at 09:15

How to return just one string from java stream

2 Answers2