Mapping one object to another with aggregation and grouping

user1298426 :

Mapping one object to another with aggregation and grouping

I have a class

class Q {
    String username;
    int taskId;
    double timediff;

   // constructor
}

and I need to convert it to

 class ToQ {
        String username;
        int noOfTasks;
        double sum;

       // constructor
    }

So the intension is group by username, count no of taskIds, sum of timediff.

List of this

List<Q> qs = new ArrayList<>();
    qs.add(new Q("tom", 2, 2.0));
    qs.add(new Q("tom", 6, 1.0));
    qs.add(new Q("harry", 8, 0.03));

Should have output of ToQ as

new ToQ("tom",2,3);
new ToQ("harry",1,0.03);

I tried with grouping function but it generates HashMap but not sure how to convert into object.

Map<String, Double> collect = qs
            .stream()
            .collect(groupingBy(Q::getUsername, summingDouble(Q::getLocalDateTime)));

Naman :

You could do it as follows:

Group by Name for TimeDiff

Map<String, Double> nameToTimeDiffSumMap = qs.stream()
        .collect(Collectors.groupingBy(Q::getUsername, 
                Collectors.summingDouble(Q::getTimediff)));

Group by Name for Count

Map<String, Long> nameToCountMap = qs.stream()
        .collect(Collectors.groupingBy(Q::getUsername, 
                Collectors.counting()));

Form a List<ToQ>

List<ToQ> toQS = qs.stream().map(a -> 
        new ToQ(a.getUsername(), nameToCountMap.get(a.getUsername()), nameToTimeDiffSumMap.get(a.getUsername())))
        .collect(toList());

which assumes the classes to have getters and setters and appropriate constructor as well

class ToQ {
    String username;
    long noOfTasks;
    double sum;
}

class Q {
    String username;
    int taskId;
    double timediff;
}