Initial data commit

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

@@ -0,0 +1,28 @@
+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}));
+    }
+}