Partitioning a Map in Java 8+

Question

I have a Map<String, String> and a List<String>. I'd like to partition the Map based on the condition

foreach(map.key -> list.contains(map.key))

and produce two Map(s). What's the most elegant way to do so? I'm on Java 11, so you can throw everything you want in the answers.

What I came up to for now is:

map.entrySet()
   .stream()
   .collect(partitioningBy(e -> list.contains(o.getKey())));

but that gives a Map<Boolean, List<Entry<String, String>>>.

Answer 1

You can reduce each group using toMap (as a downstream collector):

Map<String, String> myMap = new HashMap<>(); myMap.put("d", "D"); myMap.put("c", "C"); myMap.put("b", "B"); myMap.put("A", "A");  List<String> myList = Arrays.asList("a", "b", "c");  Map<Boolean, Map<String, String>> result = myMap.entrySet()         .stream()         .collect(Collectors.partitioningBy(                             entry -> myList.contains(entry.getKey()),                             Collectors.toMap(Entry::getKey, Entry::getValue)                     )         ); 

And for this example, that produces {false={A=A, d=D}, true={b=B, c=C}}

Answer 2

Though partitioningBy is the way to go when you need both the alternatives as an output based on the condition. Yet, another way out (useful for creating map based on a single condition) is to use Collectors.filtering as :

Map<String, String> myMap = Map.of("d", "D","c", "C","b", "B","A", "A");
List<String> myList = List.of("a", "b", "c");
Predicate<String> condition = myList::contains;

Map<String, String> keysPresentInList = myMap.keySet()
        .stream()
        .collect(Collectors.filtering(condition,
                Collectors.toMap(Function.identity(), myMap::get)));
Map<String, String> keysNotPresentInList = myMap.keySet()
        .stream()
        .collect(Collectors.filtering(Predicate.not(condition),
                Collectors.toMap(Function.identity(), myMap::get)));

or alternatively, if you could update the existing map in-place, then you could retain entries based on their key's presence in the list using just a one-liner:

myMap.keySet().retainAll(myList);