how to convert a hashmap of list into a single list in Java 8?

Question

I am completely new to java 8 and i am a bit unclear on how to proceed. i have a Map <String, List<value>> in Java 7, i would just use for loop on the keys and collect the List into a single list.

however, i want to be able to do this in 8. what i have is:

List<Value> newList = resultMap.entrySet().stream()
                       .flatMap( e -> e.getValue().stream())
                       .map( // get the value in the list)
                       .collect(Collectors.toList())

However, in this case, i would not be able to know the key from the hashmap which the value belongs to.

How can i get the value of the key for the hashmap while doing the above?

Answer 1

You can do something like this:

Map<Key, List<Value>> map = ...;

List<Map.Entry<Key, Value>> list =
    map.entrySet()
        .stream()
        .flatMap(e -> {
          return e.getValue().stream()
              .map(v -> new AbstractMap.SimpleEntry<>(e.getKey(), v));
        })
        .collect(Collectors.toList());

This creates an entry for each value in each sublist, where the key is the corresponding key for the list that value came from.