Averaging across multiple fields with IntSummaryStatistics

Question

I\'m trying to use Java 8 streams to create a single CarData object, which consists of an average of all the CarData fields in the list coming from getCars;

Answer 1

You need to write your own Collector, something like this:

class CarDataAverage {
    public static Collector> get() {
        return Collector.of(CarDataAverage::new, CarDataAverage::add,
                            CarDataAverage::combine,CarDataAverage::finish);
    }
    private long sumBodyWeight;
    private long sumShellWeight;
    private int count;
    private void add(CarData carData) {
        this.sumBodyWeight += carData.getBodyWeight();
        this.sumShellWeight += carData.getShellWeight();
        this.count++;
    }
    private CarDataAverage combine(CarDataAverage that) {
        this.sumBodyWeight += that.sumBodyWeight;
        this.sumShellWeight += that.sumShellWeight;
        this.count += that.count;
        return this;
    }
    private Optional finish() {
        if (this.count == 0)
            return Optional.empty();
        // adjust as needed if averages should be rounded
        return Optional.of(new CarData((int) (this.sumBodyWeight / this.count),
                                       (int) (this.sumShellWeight / this.count)));
    }
}

You then use it like this:

List list = ...

Optional averageCarData = list.stream().collect(CarDataAverage.get());