How to convert a Collection<Pair<K, Collection<V>> to a List<MyObject<K,V>> using Java stream API?

Question

My traditional code would look like this:

List<MyObject> transform(Collection<java.util.Map.Entry<String, List<String>>> input) {
    List<MyObject> output = new LinkedList<>();
    for (Entry<String, List<String>> pair : input) {
        for (String value : pair.getValue()) {
            output.add(new MyObject(pair.getKey(), value));
        }
    }
    return output;
}

Can I do the same with lambda expressions? I’ve tried around, but I don’t get it. The outer collection is unsorted, but the List<String> is sorted. The result objects may return in the result list without any order, with the exception that objects created from the same key String should follow each other to preserve the order of the value. Is this at all possible?

Answer 1

input.stream()
     .flatMap(e -> e.getValue()
                    .stream()
                    .map(v -> new MyObject(e.getKey(), v)))
     .collect(Collectors.toCollection(LinkedList::new));