user1990218 :
I have a Map<String, Set<String>>
that I need to reverse. This would be an example of data:
("AA", ("AA01", "AA02", "AA03")
("BB", ("AA01", "BB01", "BB02")
What I want to obtain is a Map<String, Set<String>>
of the reverse relationship, something like this:
("AA01", ("AA", "BB"))
("AA02",("AA"))
("AA03",("AA"))
("BB01",("BB"))
("BB02",("BB"))
I was able to do it, but using foreach:
private Map<String, Set<String>> getInverseRelationship(Map<String, Set<String>> mappings) {
Map<String, Set<String>> result = new HashMap<>();
mappings.entrySet().stream().forEach(record -> {
String key = record.getKey();
Set<String> itemSet = record.getValue();
itemSet.forEach(item -> {
Set<String> values = (result.containsKey(item))? result.remove(item) : new HashSet<>();
values.add(key);
result.put(item, values);
});
});
return result;
}
Is there a better way to do it using Java 8 streams API?
Ruslan :
You could use flatMap
like :
Map<String, Set<String>> invertedMap = map.entrySet().stream()
.flatMap(entry -> entry.getValue().stream()
.map(v -> new AbstractMap.SimpleEntry<>(v, entry.getKey())))
.collect(groupingBy(Map.Entry::getKey, mapping(Map.Entry::getValue, toSet())));
Using SimpleEntry
you can store each element from Set
as an entry's key and map's key as an entry's value