limit(long) difference order in a pipeline

octopus :

What is the difference between

stream.map().limit(5).terminalOperation()

and this line

stream.limit(5).map().terminalOperation()

the both lines will return the same result, but which one is more optimized and will not consume more time and resources ? and why ? and how many times the map will be called in each case?

Eran :

how many times the map will be called in each case?

The answer depends on the terminal operation, not on the order of map and limit.

The terminal operation may consume just one element (findFirst()) or all elements (collect()).

Either way, the terminal operation dictates the number of elements on which map is performed, which should be identical in both cases.

For example, if you collect the elements to a List, assuming the Stream source has at least 5 elements, map will be executed 5 times for each pipeline.

You can verify that by adding a println statement to the map step:

List<String> list = Arrays.asList ("a","b","c","d","e","f");
list.stream().map(l->{System.out.println ("map1 " + l);return l +"44";}).limit(5).collect(Collectors.toList());
list.stream().limit(5).map(l->{System.out.println ("map2 " + l);return l +"44";}).collect(Collectors.toList());

This will output:

map1 a
map1 b
map1 c
map1 d
map1 e
map2 a
map2 b
map2 c
map2 d
map2 e

Either way, map is not executed for the 6th element.

As to which variant to prefer, I'm assuming they'll have similar running time, but that depends on the actual implementation.