How to clear the contents of a list which is inside a HashMap value using Java 8 stream

Question

I have a HashMap with the following structure. Map<String, Container>

The Container class contains a List. I want to clear the contents of this list so that the list exists but with 0 elements. Later I will put values in it again.

The replaceAll() expects a BiFunction. Due to this the following is giving compilation error because return type of clear() is void:

personMap.replaceAll((k,v) -> v.getMyList().clear());

Answer 1

You can use this:

map.values().forEach(i -> i.getMyList().clear());

Alternatively you can use this:

map.values().stream()
        .map(Container::getMyList)
        .forEach(List::clear);

Answer 2

Iterating your Map with forEach will require a BiConsumer<? super K, ? super V> (where K and V are the key and value types of your Map), which will ultimately allow you to invoke void methods on the elements.

E.g.

personMap.forEach((k,v) -> v.getMyList().clear());