How to flatten a list inside a map in Java 8

Question

How can I go from a map of integers to lists of strings such as:

<1, ["a", "b"]>,
<2, ["a", "b"]>

To a flattened list of strings such as:

["1-a", "1-b", "2-a", "2-b"]

in Java 8?

Answer 1

You can use flatMap on values as:

map.values()
   .stream()
   .flatMap(List::stream)
   .collect(Collectors.toList());

Or if you were to make use of the map entries, you can use the code as Holger pointed out :

map.entries()
   .stream()
   .flatMap(e -> e.getValue().stream().map(s -> e.getKey() + s))
   .collect(Collectors.toList());