How to sum value in same key HashMap Java?

Question

I need to sum the value in same key not just to replace? Thanks.

    LinkedHashMap<String, LinkedHashMap<String, Integer>> users = new LinkedHashMap<>();
    for (int i = 0; i < n; i++) {
        String[] input = scanner.readLine().split("\\s+");
        String ip = input[0];
        String name = input[1];
        int duration = Integer.parseInt(input[2]);
        if (!users.containsKey(name)) {
            users.put(name, new LinkedHashMap<>());
            users.get(name).put(ip,duration);
        } else {
            users.get(name).put(ip,duration);
        }
    }

Use a Map<String, YourClass>, where YourClass has a name, an ip, and a cumulatedDuration. Add an addDuration(int duration) method to the class to add a duration to the cumulated duration. Java is an OO language. Use classes. This will make your code much clearer (and faster, too). — JB Nizet
– JB Nizet, Commented Dec 29, 2017 at 15:56
Thanks for your advice but the aim is to solve without classes and I am trying :) — Anton Stoychev
– Anton Stoychev, Commented Dec 29, 2017 at 16:02

fps · Accepted Answer · 2017-12-29 19:32:54Z

3

You can do it by using Map.computeIfAbsent in the outer map, along with Map.merge in the inner maps:

Map<String, Map<String, Integer>> users = new LinkedHashMap<>();

for (int i = 0; i < n; i++) {
    String[] input = scanner.readLine().split("\\s+");
    String ip = input[0];
    String name = input[1];
    int duration = Integer.parseInt(input[2]);

    users.computeIfAbsent(name, k -> new LinkedHashMap<>())
        .merge(ip, duration, Integer::sum);
}

answered Dec 29, 2017 at 19:32

fps

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1 Comment

Anton Stoychev Over a year ago

You are god, I was waiting exactly about this answer. Simply I wanna sum by names not IP because one user can in by different IPs. Big thanks!

Mykyta Lebid · Accepted Answer · 2017-12-29 16:06:25Z

2

I believe it'd be your answer Try this:

for (int i = 0; i < n; i++) {
        String[] input = scanner.readLine().split("\\s+");
        String ip = input[0];
        String name = input[1];
        int duration = Integer.parseInt(input[2]);
        if (!users.containsKey(name)) {
            users.put(name, new LinkedHashMap<>());
            users.get(name).put(ip, duration);
        } else {
            LinkedHashMap<String, Integer> user = users.get(name);
            Integer userDuration = user.get(ip);
            user.put(ip, userDuration + duration);
        }
    }

answered Dec 29, 2017 at 16:06

Mykyta Lebid

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3 Comments

Jesse Nelson Over a year ago

Not sure of the setup here but if there is ever the same name and different ip this will throw a null pointer

Anton Stoychev Over a year ago

Yes you are right "Exception in thread "main" java.lang.NullPointerException"

Jesse Nelson Over a year ago

I have an answer above that should work for you in this case

marc_s · Accepted Answer · 2023-08-10 20:10:48Z

LinkedHashMap<String, LinkedHashMap<String, Integer>> users = new LinkedHashMap<>();
    for (int i = 0; i < n; i++) {
        String[] input = scanner.readLine().split("\\s+");
        String ip = input[0];
        String name = input[1];
        int duration = Integer.parseInt(input[2]);
        LinkedHashMap<String, Integer> ipDurations= users.get(name);
        
        if (ipDurations== null) {
            ipDurations= new LinkedHashMap<>();
            ipDurations.put(ip, duration);
            users.put(name, ipDurations);
        } else {
            Integer cummulativeDuration = ipDurations.get(ip);
            if (cummulativeDuration == null) {
               cuumulativeDuration = 0;
               ipDurations.put(ip, cummulativeDuration);
            }
            cummulativeDuration += duration;
          }
     }

Oleg Zinoviev · Accepted Answer · 2017-12-29 16:28:19Z

1

Unfortunately I cannot add full snippet due I type from phone, but I beleive all you need is stream and collect with merge if you need initial map later, or just merge instead put if you dont

answered Dec 29, 2017 at 16:28

Oleg Zinoviev

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Comments

Pace · Accepted Answer · 2017-12-29 15:55:30Z

0

int oldDuration = users.get(name).get(ip);
int totalDuration = oldDuration + duration;
users.get(name).put(ip, totalDuration);

answered Dec 29, 2017 at 15:55

Pace

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