how to get multi properties's mean values using java8 stream? - java-8

Question

there's one Object
#Data
class ScoreInfo{
String id;
float cove_score;
float theam_score;
float content_score;
float teach_score;
Date create_date;
ScoreInfoP scoreInfoP;
}
and ScoreInfoP is :
#Data
class ScoreInfoP{
String stream_sn;
String anchor_id;
String create_by;
}
sourceList is a list of ScoreInfo,I want to get cove_score,theam_score,content_score,teach_score's mean values,group by scoreInfoP property and return four mean values for each of these properties.
I can get only one mean value using such code:
Map<ScoreInfoP, Double> meanForCoveScore = sourceList.stream().collect(Collectors.groupingBy(ScoreInfo::getScoreInfoP,
Collectors.averagingDouble(ScoreInfo::getCove_score)));
I want to learn how to get four mean values using java8 or any easier way you suggest achieving this.
Waiting here for your generous help.

Answer 1

There isn't anything build-in for this, but it's not that complicated to build a custom Collector for that...
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);
I actually groupBy here on ScoreInfoP#anchorId; but you can do it on ScoreInfoP - for that you need to change x -> x.getScoreInfoP().getAnchorId() to x -> x.getScoreInfoP(). But obviously ScoreInfoP needs to override hashCode and equals.

Answer 2

As I said in the comment you should use a proper result class.
class ScoreInfoAverage {
private float cove_score;
private float theam_score;
private float content_score;
private float teach_score;
// ctor, getter, setter
}
Then you can use a custom 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);
}
Last but not least you add your Collector to the downstream:
Map<ScoreInfoP, ScoreInfoAverage> collect = scoreInfos.stream()
.collect(Collectors.groupingBy(ScoreInfo::getScoreInfoP, scoreInfoToAverage()));