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

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

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.

Solution

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())))));