Java 8 - stream convert map's value type

Question

I would like to convert type List<A> to List<B>. can I do this with java 8 stream method?

    Map< String, List<B>> bMap = aMap.entrySet().stream().map( entry -> {
        List<B> BList = new ArrayList<B>();
        List<A> sList = entry.getValue();
        // convert A to B
        return ???; Map( entry.getKey(), BList) need to return
    }).collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));

I tried with this code, but cannot convert it inside map().

Answer 1

If I understood it correctly you have a Map<String, List<A>> and you want to convert it to a Map<String, List<B>>. You can do something like:

Map<String, List<B>> result = aMap.entrySet().stream()
    .collect(Collectors.toMap(
        entry -> entry.getKey(),                        // Preserve key
        entry -> entry.getValue().stream()              // Take all values
                     .map(aItem -> mapToBItem(aItem))   // map to B type
                     .collect(Collectors.toList())      // collect as list
        );