How to flatten map values using java streams

Question

I am new to Java streams and have a problem at hand. I have a map like this:

Map<String, List<String>> specialProductsMap

And i want to flatten the map values to a set which contains all the String values in lists in the specialProductsMap. How can i do this using Java Streams?

Answer 1

You may use the flatMap operator to get this thing done. Here's how it looks.

Set<String> valueSet = specialProductsMap.values().stream()
    .flatMap(List::stream)
    .collect(Collectors.toSet());

Answer 2

First Obtain the list of values from map then use stream api like this

Set<String> setOfString = specialProductsMap.values().stream().flatMap(list->list.stream())
            .collect(Collectors.toSet());

Or Like this Using Method reference

Set<String> setOfString = specialProductsMap.values().stream().flatMap(List::stream)
            .collect(Collectors.toSet());