Why can't mapToInt be used with collect(toList())?

Question

I wrote this code:

import java.util.*;
import java.util.stream.Collectors;

public class Main {

    public static void main(String... args) {
        List<String> a = Arrays.asList("One", "Two", "three");
        List<Integer> lengths = a.stream().mapToInt(String::length).collect(Collectors.toList());

    }
}

but it doesn't want to compile, saying:

Error:(8, 68) java: method collect in interface java.util.stream.IntStream cannot be applied to given types;
  required: java.util.function.Supplier<R>,java.util.function.ObjIntConsumer<R>,java.util.function.BiConsumer<R,R>
  found: java.util.stream.Collector<java.lang.Object,capture#1 of ?,java.util.List<java.lang.Object>>
  reason: cannot infer type-variable(s) R
    (actual and formal argument lists differ in length)

What's happening here? Why the limitation? If you use map. Instead of mapToInt, it works fine.

Answer 1

mapToInt produces an IntStream, which doesn't have collect method taking a single Collector argument.

Since the end result is a List<Integer>, you don't have to transform the Stream to an IntStream:

List<Integer> lengths = a.stream().map(String::length).collect(Collectors.toList());

Transforming the Stream to an IntStream would make sense if you want to collect the elements of the Stream to an primitive array:

int[] lengths = a.stream().mapToInt(String::length).toArray();

If you want to transform to an IntStream and still use the collect method, you can write the following (which is less recommended):

List<Integer> lengths = 
    a.stream()
     .mapToInt(String::length)
     .collect(ArrayList::new, ArrayList::add, ArrayList::addAll);

Answer 2

After .mapToInt you got an IntStream.

The collect definition for this stream is:

<R> R collect(Supplier<R> supplier,
              ObjIntConsumer<R> accumulator,
              BiConsumer<R, R> combiner);

Hence you cannot simply do toList() here.