How to filter a map by its values in Java 8?

Question

I have a map where the values are strings and the keys are lists: Map<String, List<BoMLine>> materials. I'd like to filter this map by its values; something like this:

materials.entrySet().stream()        .filter(a->a.getValue().stream()            .filter(l->MaterialDao.findMaterialByName(l.getMaterial()).ispresent) 

But it's not working for me. Does anybody have an idea?

Answer 1

If I understand your filtering criteria correctly, you want to check if the filtered Stream you produced from the value List has any elements, and if so, pass the corresponding Map entry to the output Map.

Map<String, List<BoMLine>>     filtered = materials.entrySet()                         .stream()                         .filter(a->a.getValue()                                     .stream()                                     .anyMatch(l->MaterialDao.findMaterialByName(l.getMaterial())))                         .collect(Collectors.toMap(e->e.getKey(),e->e.getValue())); 

This is assuming MaterialDao.findMaterialByName(l.getMaterial()) returns a boolean.

Answer 2

Generally, this is how you can filter a map by its values:

static <K, V> Map<K, V> filterByValue(Map<K, V> map, Predicate<V> predicate) {     return map.entrySet()             .stream()             .filter(entry -> predicate.test(entry.getValue()))             .collect(Collectors.toMap(Entry::getKey, Entry::getValue)); } 

Call it like this:

Map<String, Integer> originalMap = new HashMap<>(); originalMap.put("One", 1); originalMap.put("Two", 2); originalMap.put("Three", 3);  Map<String, Integer> filteredMap = filterByValue(originalMap, value -> value == 2);  Map<String, Integer> expectedMap = new HashMap<>(); expectedMap.put("Two", 2);  assertEquals(expectedMap, filteredMap);