What is the easiest way to change the inner value type in a nested map in java?

Question

I have a nested map Map<String, Map<String, List<ObjectA>>> passed to me, and I want to change it to type Map<String, Map<String, Set<ObjectA>>>, what is the easiest way to do so in Java using stream? I have tried to use Collectors.groupingBy but can't get it working.

Answer 1

The best way is you have to iterate through each entry in outer map and inner map, and then convert the inner map entry value List<ObjectA> to Set<ObjectA>

Map<String, Map<String, Set<ObjectA>>> resultMap = map.entrySet().stream()
            .collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue().entrySet().stream()
                    .collect(Collectors.toMap(Map.Entry::getKey, val -> new HashSet<>(val.getValue())))));

Note : If you are converting List to HashSet then you will not maintain same order, so you can choose LinkedHashSet over HashSet to maintain order

Map<String, Map<String, Set<ObjectA>>> resultMap = map.entrySet().stream()
            .collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue().entrySet().stream()
                    .collect(Collectors.toMap(Map.Entry::getKey, val -> new LinkedHashSet<>(val.getValue())))));