RosettaCodeData/Task/Diversity-prediction-theorem/Java/diversity-prediction-theorem.java

import java.util.Arrays;

public class DiversityPredictionTheorem {
    private static double square(double d) {
        return d * d;
    }

    private static double averageSquareDiff(double d, double[] predictions) {
        return Arrays.stream(predictions)
            .map(it -> square(it - d))
            .average()
            .orElseThrow();
    }

    private static String diversityTheorem(double truth, double[] predictions) {
        double average = Arrays.stream(predictions)
            .average()
            .orElseThrow();
        return String.format("average-error : %6.3f%n", averageSquareDiff(truth, predictions))
            + String.format("crowd-error   : %6.3f%n", square(truth - average))
            + String.format("diversity     : %6.3f%n", averageSquareDiff(average, predictions));
    }

    public static void main(String[] args) {
        System.out.println(diversityTheorem(49.0, new double[]{48.0, 47.0, 51.0}));
        System.out.println(diversityTheorem(49.0, new double[]{48.0, 47.0, 51.0, 42.0}));
    }
}