如何使用java8流獲取多屬性的平均值？

Question

有一個對象

@Data 
class ScoreInfo{ 
String id; 

float cove_score; 
float theam_score; 
float content_score; 
float teach_score; 

Date create_date; 
ScoreInfoP scoreInfoP; 

}

和ScoreInfoP是：

@Data 
class ScoreInfoP{ 

String stream_sn; 

String anchor_id; 

String create_by; 
} 

SOURCELIST是ScoreInfo的名單，我想cove_score，theam_score，content_score，teach_score的平均值，組通過scoreInfoP屬性並返回每個屬性的四個平均值。

我可以用這樣的代碼只能得到一個平均值：

Map<ScoreInfoP, Double> meanForCoveScore = sourceList.stream().collect(Collectors.groupingBy(ScoreInfo::getScoreInfoP, 
      Collectors.averagingDouble(ScoreInfo::getCove_score))); 

我想學習怎樣用java8或您建議實現這一目標變得更容易雙向四個平均值。

在此等待您的慷慨幫助。

Answer 1

有沒有什麼集結在此，但它並不複雜，以建立一個定製的珍藏......

Map<String, List<Float>> result = Arrays.asList(first, second) 
      .stream() 
      .collect(Collectors.groupingBy(
        x -> x.getScoreInfoP().getAnchorId(), 
        Collector.of(
          () -> new float[5], 
          (a, x) -> { 
           a[0] += x.getCoveScore(); 
           a[1] += x.getTheamScore(); 
           a[2] += x.getTeachScore(); 
           a[3] += x.getContentScore(); 
           a[4]++; 
          }, 

          (left, right) -> { 
           for (int i = 0; i < 4; ++i) { 
            left[i] += right[i]; 
           } 

           return left; 
          }, x -> Arrays.asList(x[0]/x[4], x[1]/x[4], x[2]/x[4], x[3]/x[4])) 

    )); 

    System.out.println(result); 

我其實GROUPBY這裏ScoreInfoP#anchorId;但你可以在ScoreInfoP上做 - 因爲你需要將x -> x.getScoreInfoP().getAnchorId()更改爲x -> x.getScoreInfoP()。但顯然ScoreInfoP需要覆蓋hashCode和equals。

Answer 2

正如我在評論中所說，你應該使用適當的結果類。

class ScoreInfoAverage { 
    private float cove_score; 
    private float theam_score; 
    private float content_score; 
    private float teach_score; 

    // ctor, getter, setter 
} 

---

然後你就可以使用自定義Collector：

public static Collector<ScoreInfo, ?, ScoreInfoAverage> scoreInfoToAverage() { 
    class ScoreInfoAccumulator { 
    private DoubleSummaryStatistics cove_score = new DoubleSummaryStatistics(); 
    private DoubleSummaryStatistics theam_score = new DoubleSummaryStatistics(); 
    private DoubleSummaryStatistics content_score = new DoubleSummaryStatistics(); 
    private DoubleSummaryStatistics teach_score = new DoubleSummaryStatistics(); 

    public void add(ScoreInfo si) { 
     cove_score.accept(si.cove_score); 
     theam_score.accept(si.theam_score); 
     content_score.accept(si.content_score); 
     teach_score.accept(si.teach_score); 
    } 

    public ScoreInfoAccumulator combine(ScoreInfoAccumulator sia) { 
     cove_score.combine(sia.cove_score); 
     theam_score.combine(sia.theam_score); 
     content_score.combine(sia.content_score); 
     teach_score.combine(sia.teach_score); 
     return this; 
    } 

    public ScoreInfoAverage average() { 
     return new ScoreInfoAverage((float) cove_score.getAverage(), 
     (float) theam_score.getAverage(), (float) content_score.getAverage(), 
     (float) teach_score.getAverage()); 
    } 
    } 
    return Collector.of(ScoreInfoAccumulator::new, ScoreInfoAccumulator::add, 
     ScoreInfoAccumulator::combine, ScoreInfoAccumulator::average); 
} 

---

最後但並非最不重要的，你添加Collector下游：

Map<ScoreInfoP, ScoreInfoAverage> collect = scoreInfos.stream() 
     .collect(Collectors.groupingBy(ScoreInfo::getScoreInfoP, scoreInfoToAverage()));