collect a synchronized arraylist from streams in java 8

Question

List<String> result = map.entrySet()
                 .stream()
                     .map(Map.Entry::getValue)
                     .flatMap(x -> x.stream())
                     .collect(Collectors.toCollection(ArrayList::new));

The above code will create a ArrayList which is not thread safe. So how can be make it thread safe.

Answer 1

If you want a synchronized collection, you can just change your collector to provide the implementation you want, for example:

.collect(Collectors.toCollection(() -> Collections.synchronizedList(new ArrayList<> ()));

Or if you prefer a concurrent collection:

.collect(Collectors.toCollection(CopyOnWriteArrayList::new));

In the latter case, it may be more efficient to use the copy constructor to avoid unnecessary copies of the underlying array.

Answer 2

You can add a synchronized proxy over it after you create it

Collections.synchronizedList(map.entrySet()
                 .stream()
                     .map(Map.Entry::getValue)
                     .flatMap(x -> x.stream())
                     .collect(Collectors.toCollection(ArrayList::new)))

Answer 3

Slightly better is to move the wrapping into the Collector:

map.entrySet().stream()
   .map(Map.Entry::getValue)
   .flatMap(x -> x.stream())
   .collect(collectingAndThen(toList(), 
                              Collections::synchronizedList))