Java，Streams：如何使用.collect（Collectors.toList（））转换表达式

Question

How can I convert

List<Integer> list = new ArrayList<>();
IntStream.range(0, 10).forEach(i -> IntStream.range(0, i + 1).forEach(j -> list.add(Math.max(i, j))));

into a single line expression like

List<Integer> list = IntStream.range(0, 10) ... .collect(Collectors.toList());

Answer 1

Use flatMap :

IntStream.range(0, 10)
         .flatMap(i -> IntStream.range(0, i + 1).map(j -> Math.max(i, j)))
         .boxed()
         .collect(Collectors.toList());

As a side note, you don't need Math.max here. In this case i is always not less than j , so the same result can be generated simpler:

IntStream.range(0, 10)
    .flatMap(i -> IntStream.range(0, i + 1).map(j -> i))
    .boxed()
    .collect(Collectors.toList());

Answer 2

You can achieve this with flatMap :

List<Integer> list =
    IntStream.range(0,10)
             .flatMap(i -> IntStream.range(0, i + 1)
                                    .map(j->Math.max(i,j)))
             .boxed()
             .collect(Collectors.toList());

EDIT : just tested my code. It seems mapToObj doesn't work, so you have to use map and box the stream later.