How to convert List<DataObject> to Map<Integer, String> using streams

Question

I have a list of objects. I want to do the following

for (int i = 0; i < sorted.size(); i++) {
    map.put(i, sorted.get(i).getName());
}

Is there a straightforward way to do this in Java 8 streams api ?

Answer 1

You can do the following:

Map<Integer, String> map =
        IntStream.range(0, sorted.size())
                 .boxed()
                 .collect(toMap(i -> i, i -> sorted.get(i).getName()));

This creates a Stream of int going from 0 to the size of list and collects each value into a Map.

Answer 2

Since you want to collect the elements into a data structure which intrinsically tracks the position as its size, you can do the operation using a custom collector:

Map<Integer, String> map=sorted.stream()
    .map(x -> x.getName())
    .collect(HashMap::new, (m,s) -> m.put(m.size(), s),
        (a,b) -> { int off=a.size(); b.forEach((i,s) -> a.put(i+off, s));});

The trickiest part is the merge function which will be used in parallel processing only. It adds all elements of a map to its preceding map, but has to offset the keys by the number of preceding elements.