Get list of duplicate map keys from multiple maps

Question

I have multiple maps of arrays.

Map<String, List<String>> map1 = new HashMap<>();
Map<String, List<String>> map2 = new HashMap<>();
Map<String, List<String>> map3 = new HashMap<>();
Map<String, List<String>> map4 = new HashMap<>();

I want to get the list of duplicate map keys from the multiple maps.

For example

map1.put("k0", Arrays.asList("a0", "a1"));
map1.put("k1", Arrays.asList("b0", "b1"));

map2.put("k1", Arrays.asList("z1", "z2"));
map2.put("k2", Arrays.asList("z1", "z2"));

map3.put("k1", Arrays.asList("z1", "z2"));
map3.put("k3", Arrays.asList("z1", "z2"));

map4.put("k3", Arrays.asList("z1", "z2"));
map4.put("k4", Arrays.asList("z1", "z2"));

map5.put("k0", Arrays.asList("z1", "z2"));
map5.put("k5", Arrays.asList("z1", "z2"));

// Expected output is 
List: [k0, k1, k3]

Aside from iterating through all of the map keys, checking if a set contains the key, and if not adding the key to the set, I can't think of any cleaner ways of doing this. Is there a way to accomplish this with streams?

you don't need to check if Set contains the key as Set items are unique. Just iterate throw the all keys, add them to the set and use the result. — Sergey Grinev, Dec 16 '20 at 07:00

Youcef LAIDANI · Answer 1 · 2020-12-16T07:46:34.403

2

Because you are using java-stream tag, you can solve your problem like so:

Set<String> duplicateKeys = Stream.of(map1.keySet(), map2.keySet(), map3.keySet(), map4.keySet(), map5.keySet())
        .flatMap(Set::stream)
        .collect(Collectors.toMap(Function.identity(), x -> false, (a, b) -> true))
        .entrySet().stream()
        .filter(Map.Entry::getValue)
        .map(Map.Entry::getKey)
        .collect(Collectors.toSet());

Output

[k0, k1, k3]

.collect(Collectors.toMap(Function.identity(), x -> false, (a, b) -> true)) this return a Map<String, Boolean> it return the key and true if duplicate and false if not, then you just filter to get only the entries where the value is true .filter(Map.Entry::getValue) which represent the duplicate ones.

edited Dec 16 '20 at 07:46

answered Dec 16 '20 at 07:12

Youcef LAIDANI

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Wow, thank you. Can you explain a little bit how this works, especially the "Collectors.toMap(Function.identity(), x -> false, (a, b) -> true)"? – villabilla Dec 16 '20 at 07:30
@villabilla Yes It is a trick to collect as a Map and in the value part you just use a true for duplicate and false for not duplicate – Youcef LAIDANI Dec 16 '20 at 07:48
Can you please explain how the following part works (technically) Collectors.toMap(Function.identity(), x -> false, (a, b) -> true) – Panagiotis Bougioukos Dec 16 '20 at 08:34
@PanagiotisBougioukos the `x -> false` mean if there are a unique element then return false, the `(a, b) -> true` mean if a and b are equals this mean there are duplicate so return true, for more details [read this](https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html#toMap-java.util.function.Function-java.util.function.Function-java.util.function.BinaryOperator-java.util.function.Supplier-) – Youcef LAIDANI Dec 16 '20 at 08:50

score 2 · Accepted Answer · answered Dec 16 '20 at 07:37

@SafeVarargs
private static Set<String> findDuplicateKeys(Map<String, ?>... maps) {
    Set<String> keys = new HashSet<>();
    return Arrays.stream(maps)
            .flatMap(map -> map.keySet().stream())
            .filter(key -> !keys.add(key))
            .collect(Collectors.toSet());
}

I would model the result as a set rather than list, since no duplicate elements are possible.

score 0 · Answer 3 · answered Dec 16 '20 at 08:42

Or, you can try this. Create the stream of maps, then flatMap it to a stream of keys, then collect the keys into a Map counting how many of each key there is, then stream that EntrySet, Set<Map.Entry>, filter by the count of the occurrence of the keys, map that to the key values and collect to a list. This should be able to run with .parallel().

 private List<String> findDups(Map<String, ?>... maps) {
 return Arrays.asList(maps).stream()
  .flatMap(m->m.keySet().stream())
  .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
  .entrySet().stream()
  .filter(es->es.getValue() > 1)
  .map(es->es.getKey())
  .collect(Collectors.toList());
}

Get list of duplicate map keys from multiple maps

3 Answers3