How to filter map of map using java stream

Question

How can I filter map of map using java stream. I can do it using double loop but I think this is not efficient.

public class MapUsage {
    static Map<Integer, Map<Integer, String>> parentMap = new HashMap<>();
    static Map<String, String> userRole = new HashMap<>();

    public static void main(String ... args){
        initializeData();
        displayMapForUser("user1", parentMap);
//        printMap(parentMap);

    }


    private static void displayMapForUser(String user1, Map<Integer, Map<Integer, String>> parentMap) {
        Integer role = new Integer(userRole.get(user1));
        Map<Integer, Map<Integer, String>> userMap = new HashMap<>();
        Map<Integer, String> childMap = new HashMap<>();
        for(Map.Entry<Integer, Map<Integer, String >> entry : parentMap.entrySet()){
            for(Map.Entry<Integer, String > entry1: entry.getValue().entrySet()){
                if(entry1.getKey().equals(role))
                    childMap.put(entry1.getKey(), entry1.getValue());
            }
            userMap.put(entry.getKey(), childMap);
        }
        printMap(userMap);
    }

    private static void printMap(Map<Integer, Map<Integer, String>> parentMap) {
        for(Map.Entry<Integer, Map<Integer,String> > entry: parentMap.entrySet()){
            System.out.println("key: "+entry.getKey());
            System.out.println("value: "+entry.getValue());
        }
    }

    private static void initializeData() {
        Map<Integer, String > childMap1 = new HashMap<>();
        Map<Integer, String > childMap2 = new HashMap<>();
        userRole.put("user1", "1");
        userRole.put("user2", "2");
        userRole.put("user3", "3");
        userRole.put("user4", "4");

        childMap1.put(1, "one");
        childMap1.put(2, "two");
        childMap1.put(3, "three");
        childMap1.put(4, "four");
        parentMap.put(1, childMap1);

        childMap2.put(1, "one");
        childMap2.put(2, "two");
        childMap2.put(3, "three");
        parentMap.put(2, childMap2);
    }
}

Output is:

key: 1
value: {1=one}
key: 2
value: {1=one}

What do you mean by "filter"? As in only including members of the 2nd map that match some criterion? — Steve H.
– Steve H., Commented Feb 25, 2020 at 16:21
Yes. I would like to return the result of inner map based on the role of user. — Ashish
– Ashish, Commented Feb 25, 2020 at 16:31

Naman · Accepted Answer · 2020-02-25 16:30:50Z

5

You could use Collectors.toMap there as:

Map<Integer, Map<Integer, String>> userMap = parentMap.entrySet().stream()
        .collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue()
                .entrySet().stream()
                .filter(en -> en.getKey().equals(role))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue))));

But note, there is no significant performance efficiency that you gain here. Just readability and you could have possibly written that as :

Map<Integer, Map<Integer, String>> usersMap = new HashMap<>(parentMap);
usersMap.values().forEach(innerMap -> innerMap.entrySet().removeIf(en -> !en.getKey().equals(role)));

edited Feb 25, 2020 at 16:30

answered Feb 25, 2020 at 16:25

Naman

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1 Comment

Wiingreen Over a year ago

I would note that this has about the same complexity as the original solution, but the increased readability is a good improvement.

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