Java 8 eagerly get the result of an intermediate stream operation

Question

Given the following code:

List<String> list = Arrays.asList("a", "b", "c");
        list.stream()
        .map(s -> s + "-" + s)                 //"a-a", "b-b", "c-c"
        .filter(s -> !s.equals("b-b"))         //"a-a", "c-c"
        .forEach(s -> System.out.println(s));

map and filter are intermediate operations and forEach is a terminal operation. Only after the execution of the terminal operation we can have the result of the data transformation.

Is there any way to force the evaluation to be more eager and to have some kind of intermediate result - without breaking the stream operations chain? For example I want to have the list of "a-a", "b-b", "c-c" (which would be the result of the first intermediate operation).

Answer 1

You can use peek:

List<String> allPairs = new ArrayList<>();
List<String> list = Arrays.asList("a", "b", "c");
    list.stream()
    .map(s -> s + "-" + s)                 //"a-a", "b-b", "c-c"
    .peek(allPairs::add)
    .filter(s -> !s.equals("b-b"))         //"a-a", "c-c"
    .forEach(s -> System.out.println(s));

This way the computation still won't start until the terminal operation, but you can "intercept" the stream content at any point and use it in any way you like.

Beware however if your terminal operation is short-circuiting (like findFirst): this way not all the elements might be passed to peek.

Answer 2

Well ... if I understand your question correctly, you have to apply a terminal operation, before filtering by the not equals "b-b" predicate. Then, you should call .stream() on the intermediate result and do the filtering:

List<String> list = Arrays.asList("a", "b", "c");
list.stream()
    .map(s -> s + "-" + s)           //"a-a", "b-b", "c-c"
    .collect(Collectors.toList())    //intermediate result
    .stream()
    .filter(s -> !s.equals("b-b"))   //"a-a", "c-c"
    .forEach(s -> System.out.println(s));