How to use a lambda to filter a boolean array in Java

Question

I want to split a string by "||" and count how many elements from the resulting array return true for a function boolean isValid(String[] r).

I'm trying to use Arrays.stream to evaluate and filter the array, and finally, filter the resulting array to only keep the true values.

boolean[] truthSet = Arrays.stream(range.split("\\s+))
                           .map(r -> isValid(r))
                           .filter(b -> _whatGoesHere_)
                           .toArray(boolean[]::new);

In place of _whatGoesHere, I tried putting b, b == true, to no avail. What's the right way to achieve this?

@SharonBenAsher Well, I may be prematurely assuming, but my IDE thinks its a "bad return type in lambda expression. Object cannot be converted to boolean" — GothamCityRises
– GothamCityRises, Commented Aug 29, 2019 at 6:30
The error is not caused by b -> b. You should read it. And post it. It has to do with trying to make an array of boolean (primitive types) from a Stream<java.lang.Boolean>(objects). What's the point in creating an array with only true values anyway? Wouldn't the size of the stream convey the same information? (i.e. there are N valid strings)? — JB Nizet
– JB Nizet, Commented Aug 29, 2019 at 6:34
No need to map the elements; you can use filter directly: Arrays.stream(...).filter(str -> isValid(str)).count(). — MC Emperor
– MC Emperor, Commented Aug 29, 2019 at 8:42

Andronicus · Accepted Answer · 2019-08-29 06:43:55Z

2

You can simply pass itself, the lambda would look like b -> b (because it's already a boolean:

 Boolean[] truthSet = Arrays.stream(range.split("\\s+"))
                       .map(r -> isValid(r))
                       .filter(b -> b)
                       .toArray(Boolean[]::new);

But you're getting only values, that are true and packing them to array - this will produce a boolean array with nothing but true values. Maybe you meant:

 String[] validStrings = Arrays.stream(range.split("\\s+"))
                       .filter(r -> isValid(r))
                       .toArray(String[]::new);

P.S.: To count the values, you can use Stream#count.

edited Aug 29, 2019 at 6:43

answered Aug 29, 2019 at 6:35

Andronicus

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9 Comments

TomekK Over a year ago

First part doesn't answer this question at all.

vlumi Over a year ago

.toArray(boolean[]::new); won't work, since the type at that point is Stream<Boolean>. You could use Boolean instead of the primitive type...

Sharon Ben Asher Over a year ago

that will return an array with all "true" - so what's the difference?

GothamCityRises Over a year ago

Yeah, I meant to convert it to an array of booleans, but this works too. I Eventually want to count the number of items that returned true for isValid, so this works too. Although, I am curious, if I DID want a boolean array, how would I do it?

Sharon Ben Asher Over a year ago

@GothamCityRises, to count the number of items you don't need to collect to array, you could use Stream.count() or Collectors.counting() methods. you need to edit the question and state what is the precise requirement

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Amit Bera · Accepted Answer · 2019-08-29 06:51:55Z

0

If you want just count then you can use simple Stream#count

long count = Arrays.stream(range.split("\\s+"))
                       .map(r -> isValid(r))
                       .filter(b -> b)
                       .count();

And Since there no BooleanSteram like IntStream, DoubleStream etc,so you cann't convert Stream<Boolean> to boolean[], in that case, you have to use Boolean[]

Boolean[] truthSet = Arrays.stream(range.split("\\s+"))
                           .map(r -> isValid(r))
                           .filter(b -> b)
                           .toArray(Boolean[]::new);

edited Aug 29, 2019 at 6:51

answered Aug 29, 2019 at 6:49

Amit Bera

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2 Comments

GothamCityRises Over a year ago

filter(b -> b) would compile?

Amit Bera Over a year ago

Yes, it will compile and works fine. I believe your isValid(r) method returns a boolean value

belbix · Accepted Answer · 2019-08-29 07:39:58Z

0

I hope this is what you want

   Boolean[] truthSet = Arrays.stream(range.split("\\s+"))
            .filter(r -> isValid(r))
            .map(s -> Boolean.TRUE)
            .toArray(Boolean[]::new);

answered Aug 29, 2019 at 7:39

belbix

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Comments

Martin van Wingerden · Accepted Answer · 2019-08-29 08:29:43Z

0

If I read the question correctly the OP asks to split by '||' and is looking for the count, so a (minimal) viable solution would look like this:

import java.util.regex.Pattern;

class Scratch {
    public static void main(String[] args) {
        String test = "||a||a||b||a||bn";

        long count = Pattern.compile("\\|\\|")
                .splitAsStream(test)
                .filter(Scratch::isValid)
                .count();

        System.out.println(count);
    }

    private static boolean isValid(String r) {
        return "a".equals(r);
    }
}

answered Aug 29, 2019 at 8:29

Martin van Wingerden

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