Java 8 流操作执行顺序

Java 8 stream operations execution order(Java 8 流操作执行顺序)

问题描述

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8);
List<Integer> twoEvenSquares = numbers.stream().filter(n -> {
    System.out.println("filtering " + n);
    return n % 2 == 0;
}).map(n -> {
    System.out.println("mapping " + n);
    return n * n;
}).limit(2).collect(Collectors.toList());


for(Integer i : twoEvenSquares)
{
    System.out.println(i);
}

when executed the logic below output came

filtering 1
filtering 2
mapping 2
filtering 3
filtering 4
mapping 4
4
16

if the stream follows the short circuit concept (where we use limit stream operation), then output must be like below:

filtering 1
filtering 2
filtering 3
filtering 4
mapping 2
mapping 4
4
16

because after filtering 2, we have to still find one more element to stratify limit(2), operation, so why the output is not coming like I explained?

Streams are pull-based. Only a terminal operations (like the collect) will cause items to be consumed.

Conceptually this means that collect will ask an item from the limit, limit from the map and map from the filter, and filter from the stream.

Schematically the code in your question leads to

collect
  limit (0)
    map
      filter
        stream (returns 1)
      /filter (false)
      filter
        stream (returns 2)
      /filter (true)
    /map (returns 4)
  /limit (1)
  limit (1)
    map
      filter
        stream (returns 3)
      /filter (false)
      filter
        stream (returns 4)
      /filter (true)
    /map (returns 16)
  /limit (2)
  limit (2)
  /limit (no more items; limit reached)
/collect

And this conforms to your first printout.

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