Return only true predicates in Java stream

Question

I have the following method:

public boolean isMsgOk(String msg) {...}

I want to call this method here:

public String[] returnOkMessages(String... msgs) {
    return Arrays
        .asList(messagessgs)
        .stream(message->isMsgOk(message))
        .anyMatch() ???;
}

I want to return a List of Strings for which the isMsgOk method returns true.
How can I collect them?

Note `anyMatch` returns a `boolean`, `firstMatch` returns a `String` and `toArray` returns a `String[]` — Peter Lawrey, Feb 07 '18 at 09:09
I believe it's a duplicate. There is a lot of questions about the `filter`+`collect` pattern. — Andrew Tobilko, Feb 07 '18 at 09:12

Eran · Accepted Answer · 2018-02-07T09:29:24.390

4

Use filter and then collect to a List:

return Arrays.asList(messagessgs)
             .stream()
             .filter(message->isMsgOk(message))
             .collect(Collectors.toList())

or:

return Arrays.stream(messagessgs)
             .filter(this::isMsgOk)
             .collect(Collectors.toList())

EDIT:

If the output should be an array, use:

return Arrays.stream(messagessgs)
             .filter(this::isMsgOk)
             .toArray(String[]::new)

edited Feb 07 '18 at 09:29

answered Feb 07 '18 at 08:57

Eran

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It's not working because of the "Collectors.toList()". It says: "toList( ) in Collectors cannot be applied" – Twi Feb 07 '18 at 09:23
@Twi I noticed that the return type of your method is an array, not a List, so perhaps you should use `.toArray(String[]::new)` instead of `.collect()`. – Eran Feb 07 '18 at 09:25
I changed the return type to ArrayList but the Collectors.toList() still not working. – Twi Feb 07 '18 at 09:29
@Twi What error do you see? – Eran Feb 07 '18 at 09:30
3

@Twi change the return type to `List` – Eugene Feb 07 '18 at 09:30
Oh yes, right. I am not really awake, sorry! – Twi Feb 07 '18 at 09:31

Eugene · Answer 2 · 2018-02-07T09:26:33.220

4

Collect them to a List (you could also use a method reference instead of a lambda)

 .filter(this::isMsgOk)
 .toArray(String[]::new);

edited Feb 07 '18 at 09:26

answered Feb 07 '18 at 08:57

Eugene

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Return only true predicates in Java stream

2 Answers2