Java 8 stream operations execution order

Question

List numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8);
List twoEvenSquares = numbers.stream().filter(n -> {
    System.out.println(\"         


        

Answer 1

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.