How to collect a map key-value into a list where value is a collection

Question

Given the following map declaration Map<Integer, List<Integer>> I want to use Java 8 streams API to collect all the key-value pairs into a List<Integer> using a single stream iteration. For example, given the following mapping:

1->[2,3] 
4->[7,8]

The resulting list would be: [1,2,3,4,7,8]

@AndyTurner I need to get the keys into the list as well, not just the values. — Daniel Nitzan, Jul 26 '16 at 19:36
I probably would do it imperative: `List res = new ArrayList<>(); map.forEach((key, value) -> res.add(key); res.addAll(value);});` — eckes, Jul 26 '16 at 19:36

score 4 · Accepted Answer · answered Jul 26 '16 at 19:36

4

List<Integer> list = map.entrySet()
        .stream()
        .flatMap(e -> Stream.concat(Stream.of(e.getKey()), e.getValue().stream()))
        .collect(Collectors.toList());

answered Jul 26 '16 at 19:36

shmosel

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How to collect a map key-value into a list where value is a collection

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