[英]Group and calculate average of object array using stream in Java
I have an array of PricingSamples objects : 我有一个PricingSamples对象数组:
public class PricingSample {
private Date time;
private Double average;
public PricingSample(Date time, Double average) {
this.time = time;
this.average = average;
}
public Date getTime() {
return time;
}
public void setTime(Date time) {
this.time = time;
}
public Double getAverage() {
return average;
}
public void setAverage(Double average) {
this.average = average;
}
}
The samples are hourly for the past month. 在过去一个月中,每小时采样一次。 I would like to calculate the overall average for each day.
我想计算每天的总体平均值。 I am able to count the number of occurencies in a stream and I am also able to count the sum of all the averages for each day, I just don't know how to combine those to things into one stream.
我能够计算出一个流中出现的次数,也能够计算出每天所有平均值的总和,我只是不知道如何将这些平均值组合成一个流。
Here is what I got to. 这就是我要做的。
Summing all averages for each day (I group by the substring of the Date property so it groups by day ie Thu Apr 26): 汇总每天的所有平均值(我按Date属性的子字符串分组,因此按日期分组,即4月26日星期四):
Map<String, Double> counts = Arrays.stream(samples).collect(Collectors.groupingBy((PricingSample e) -> {
return e.getTime().toString().substring(0, 10);
},
Collectors.summingDouble((PricingSample e) -> {
return e.getAverage();
})));
Getting the number of samples for each day: Map counts = 获取每天的样本数量:图计数=
Arrays.stream(samples).collect(Collectors.groupingBy((PricingSample e) -> {
return e.getTime().toString().substring(0, 10);
},
Collectors.counting()
));
If you want the average, use averagingDouble
. 如果需要平均值,请使用
averagingDouble
。 This way you don't need to calculate the sum and the number of samples. 这样,您无需计算总数和样本数。
Map<String, Double> averages =
Arrays.stream(samples)
.collect(Collectors.groupingBy(e -> e.getTime().toString().substring(0, 10),
Collectors.averagingDouble(PricingSample::getAverage)));
If you wish to sort the Map
by its keys (the days), you can force the output to be a TreeMap
: 如果您希望按
Map
的键(日期)对Map
进行排序,则可以将输出强制为TreeMap
:
Map<String, Double> averages =
Arrays.stream(samples)
.collect(Collectors.groupingBy(e -> e.getTime().toString().substring(0, 10),
TreeMap::new,
Collectors.averagingDouble(PricingSample::getAverage)));
Try this out, 试试看
final Map<Date, Double> averageByDate = pricingSamples.stream().collect(
Collectors.groupingBy(PricingSample::getTime, Collectors.averagingDouble(PricingSample::getAverage)));
In case if you are using an array, change it like so, 如果您使用的是数组,请像这样更改它,
final Map<Date, Double> averageByDate = Stream.of(pricingSamples).collect(
Collectors.groupingBy(PricingSample::getTime, Collectors.averagingDouble(PricingSample::getAverage)));
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