通过聚合和分组将一个对象映射到另一个对象

Question

通过聚合和分组将一个对象映射到另一个对象

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

   // constructor
}

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

       // constructor
    }

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));

ToQ的输出应为

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

我尝试使用分组函数，但它生成了HashMap但不确定如何转换为对象。

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

Answer 1

您可以按照以下方式执行：

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

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

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

它假设类具有getter和setter以及适当的构造函数

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

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