How to Convert a Java 8 Intstream to a List

How do I convert a Java 8 IntStream to a List?

IntStream::boxed

IntStream::boxed turns an IntStream into a Stream<Integer>, which you can then collect into a List:

theIntStream.boxed().collect(Collectors.toList())

The boxed method converts the int primitive values of an IntStream into a stream of Integer objects. The word "boxing" names the int ⬌ Integer conversion process. See Oracle Tutorial.

Java 16 and later

Java 16 brought the shorter toList method. Produces an unmodifiable list. Discussed here.

theIntStream.boxed().toList() 

How to convert IntStream to List in java?

Explanation

Random#ints returns IntStream, not a Stream<Integer>, that is the problem.

IntStream is a special class to represent primitive ints. But generics in Java do not support primitives, i.e. there is no List<int>. So you first have to make your ints Integers, the wrapper class. Then you can collect to List<Integer>.

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Solution

Just box it and it works.

stream.boxed().toList() // since Java 16
// or
stream.boxed().collect(Collectors.toList())

Also see the documentation of IntStream#boxed:

Returns a Stream consisting of the elements of this stream, each boxed to an Integer.

For understanding, boxed() is roughly equivalent to

stream.mapToObj(x -> Integer.valueOf(x))

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So in your case:

public static void main(String[] args) {
    System.out.println(
        new Random()
            .ints(10, 0, 20)
            .boxed()
            .toList());
}

How to convert IntStream into List Character ?

You may map each int to a char then collect into a list, this will result in a list of character given by their ASCII code

List<Character> r = IntStream.range(50, 80).mapToObj(a -> (char) a).collect(Collectors.toList());
System.out.println(r); // [2, 3, 4, 5, 6, 7, 8, 9, :, ;, <, =, >, ?, @, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O]

Cannot collect Random int stream into a List collection

Before you can collect an IntStream to a List<Integer>, you need to call IntStream#boxed to transform it into a Stream<Integer>:

IntStream randIntStream = new Random().ints(1000);
List<Integer> randomInts = randIntStream.boxed()
    .collect(Collectors.toCollection(ArrayList::new));

Cannot convert IntStream to some Object Stream

The IntStream class's map method maps ints to more ints, with a IntUnaryOperator (int to int), not to objects.

Generally, all streams' map method maps the type of the stream to itself, and mapToXyz maps to a different type.

Try the mapToObj method instead, which takes an IntFunction (int to object) instead.

.mapToObj(id -> new MyObject(id));

How to populate List Integer from IntStream?

In the second example nextInt() from the Random class returns a primitive int, which can't be collected to a List. Add a call to boxed, which will convert the int's to their wrapper class Integer:

public static List<Integer> populateListStream2(int numberOfElements){
    return IntStream.range(0,numberOfElements)
                    .map(e -> random.nextInt(numberOfElements/10))
                    .boxed()
                    .collect(Collectors.toList());
}

But the first one also returned a primitive int through casting!

Yes, but it was in a Stream, so it was autoboxed to an Integer. You can tell by running:

Stream.generate(new Random()::nextDouble)
        .limit(numberOfElements)
        .map(e -> (int)(e*numberOfElements/10))
        .peek(e -> System.out.println(e.getClass()))
        .collect(Collectors.toList());

Which prints:

class java.lang.Integer

The latter was an IntStream. One of IntStream's benefits is to avoid auto boxing and unboxing. It won't box unless you explicitly call boxed()

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_{Also note that there are methods from the Random class that already return a Stream of random numbers such as ints() and doubles()}

Java8 stream - how to get convert Set to List when .map() found a Set

Use flatMap:

List<Task> taskList = projectMap.stream()
                                .flatMap(p -> p.getProject().getTasks().stream())
                                .collect(Collector.toList());

How do I filter a stream of integers into a list?

IntStream doesn't contain a collect method that accepts a single argument of type Collector. Stream does. Therefore you have to convert your IntStream to a Stream of objects.

You can either box the IntStream into a Stream<Integer> or use mapToObj to achieve the same.

For example:

return IntStream.range(0, 10)
    .filter(i -> compare(z, f(i)))
    .boxed()
    .collect(Collectors.toCollection(ArrayList::new));

boxed() will return a Stream consisting of the elements of this stream, each boxed to an Integer.

or

return IntStream.range(0, 10)
    .filter(i -> compare(z, f(i)))
    .mapToObj(Integer::valueOf)
    .collect(Collectors.toCollection(ArrayList::new));

Java stream - map and store array of int into Set

It looks like arr1 is an int[] and therefore, Arrays.stream(arr1) returns an IntStream. You can't apply .collect(Collectors.toSet()) on an IntStream.

You can box it to a Stream<Integer>:

Set<Integer> mySet = Arrays.stream(arr1)
                           .boxed()
                           .map(ele -> ele - 2)
                           .collect(Collectors.toSet());

or even simpler:

Set<Integer> mySet = Arrays.stream(arr1)
                           .mapToObj(ele -> ele - 2)
                           .collect(Collectors.toSet());

How to convert a list of objects containing other objects into HashMap

As you suspected, groupingBy is the right approach.

fruitFlowers.stream().collect(Collectors.groupingBy(
    i -> i.fruit,
    Collectors.mapping(i -> i.flower, Collectors.toList())
));

The first parameter to groupingBy defines how you want to group them. In this case, we want to group by fruits.

The second parameter describes what the value of the resulting map should be. In this case, we want the values to be the list of flowers.

Note that this approach only works if groupingBy can deduce that two fruits are equivalent. This requires that the Fruit class contains an appropriate implementation of equals and hashCode.

If the Fruit class cannot be extended to add those methods, one can change the first lambda to i -> i.fruit.name, but that would make the resulting Map to have String keys instead of Fruit.

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