Java Stream Collectors.toList() wont compile

Question

Can any one explain why the below code will not compile but the second one does?

Do not compile

private void doNotCompile() {

    List<Integer> out;
    out = IntStream
            .range(1, 10)
            .filter(e -> e % 2 == 0)
            .map(e -> Integer.valueOf(2 * e))
            .collect(Collectors.toList());

    System.out.println(out);
}

Compilation errors on the collect line

• The method collect(Supplier, ObjIntConsumer, BiConsumer) in the type IntStream is not applicable for the arguments (Collector>)
• Type mismatch: cannot convert from Collector> to Supplier

Compiles

private void compiles() {
    List<Integer> in;

    in = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);
    List<Integer> out;
    out = in.stream()
            .filter(e -> e % 2 == 0)
            .map(e -> 2 * e)
            .collect(Collectors.toList());

    System.out.println(out);
}

Answer 1

IntStream doesn't have a collect method that accepts a Collector. If you want a List<Integer>, you have to box the IntStream into a Stream<Integer>:

out = IntStream
        .range(1, 10)
        .filter(e -> e % 2 == 0)
        .map(e -> 2 * e)
        .boxed()
        .collect(Collectors.toList());

An alternative to .map().boxed() is mapToObj():

out = IntStream
        .range(1, 10)
        .filter(e -> e % 2 == 0)
        .mapToObj(e -> 2 * e)
        .collect(Collectors.toList ());

or you can use the IntStream collect method:

out = IntStream
        .range(1, 10)
        .filter(e -> e % 2 == 0)
        .map(e -> 2 * e)
        .collect(ArrayList<Integer>::new, ArrayList::add, ArrayList::addAll);