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[英]Java stream API - avoid using same predicate twice to calculate average
[英]Group and calculate average of object array using stream in Java
我有一個PricingSamples對象數組:
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;
}
}
在過去一個月中,每小時采樣一次。 我想計算每天的總體平均值。 我能夠計算出一個流中出現的次數,也能夠計算出每天所有平均值的總和,我只是不知道如何將這些平均值組合成一個流。
這就是我要做的。
匯總每天的所有平均值(我按Date屬性的子字符串分組,因此按日期分組,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();
})));
獲取每天的樣本數量:圖計數=
Arrays.stream(samples).collect(Collectors.groupingBy((PricingSample e) -> {
return e.getTime().toString().substring(0, 10);
},
Collectors.counting()
));
如果需要平均值,請使用averagingDouble
。 這樣,您無需計算總數和樣本數。
Map<String, Double> averages =
Arrays.stream(samples)
.collect(Collectors.groupingBy(e -> e.getTime().toString().substring(0, 10),
Collectors.averagingDouble(PricingSample::getAverage)));
如果您希望按Map
的鍵(日期)對Map
進行排序,則可以將輸出強制為TreeMap
:
Map<String, Double> averages =
Arrays.stream(samples)
.collect(Collectors.groupingBy(e -> e.getTime().toString().substring(0, 10),
TreeMap::new,
Collectors.averagingDouble(PricingSample::getAverage)));
試試看
final Map<Date, Double> averageByDate = pricingSamples.stream().collect(
Collectors.groupingBy(PricingSample::getTime, Collectors.averagingDouble(PricingSample::getAverage)));
如果您使用的是數組,請像這樣更改它,
final Map<Date, Double> averageByDate = Stream.of(pricingSamples).collect(
Collectors.groupingBy(PricingSample::getTime, Collectors.averagingDouble(PricingSample::getAverage)));
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