Get Index while iterating list with stream

Question

List<Rate> rateList = 
       guestList.stream()
                .map(guest -> buildRate(ageRate, guestRate, guest))
                .collect(Collectors.toList());  

class Rate {
    protected int index;
    protected AgeRate ageRate;
    protected GuestRate guestRate;
    protected int age;
}

In the above code, is it possible to pass index of guestList inside buildRate method. I need to pass index also while building Rate but could not manage to get index with Stream.

Solution 1

You haven't provided the signature of buildRate, but I'm assuming you want the index of the elements of guestList to be passed in first (before ageRate). You can use an IntStream to get indices rather than having to deal with the elements directly:

List<Rate> rateList = IntStream.range(0, guestList.size())
    .mapToObj(index -> buildRate(index, ageRate, guestRate, guestList.get(index)))
    .collect(Collectors.toList());

Solution 2

If you have Guava in your classpath, the Streams.mapWithIndex method (available since version 21.0) is exactly what you need:

List<Rate> rateList = Streams.mapWithIndex(
        guestList.stream(),
        (guest, index) -> buildRate(index, ageRate, guestRate, guest))
    .collect(Collectors.toList());

Comments

[Jason Hu]

you will need to fold(or reduce in java8). it's very annoying that java 8 implements (so-called) functional programming without providing tuple.

[Amit]

Is there Instream. range(0, size). stream() ?

[Jacob G.]

IntStream is already a Stream, so there's no need to re-stream it.

[Amit]

There is one extra .stream in your code, please remove that. I will accept your answer

[Jacob G.]

Oh whoops! I originally copied your code and forgot to remove the call to stream. My apologies.

[Holger]

To be nitpicking, IntStream is not already a Stream, but supports almost the same operations, as far as there are int specific specializations. For all other operations, you can call boxed() to truly have a Stream.