Calculate Weighted Mean in Java (#15644)

Co-authored-by: Grzegorz Piwowarek <gpiwowarek@gmail.com>

algorithms-modules/algorithms-miscellaneous-7/src/test/java/com/baeldung/algorithms/weightedaverage/WeightedAverageUnitTest.java

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+package com.baeldung.algorithms.weightedaverage;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.*;
+import java.util.function.BiConsumer;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.util.stream.Collector;
+import java.util.stream.IntStream;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class WeightedAverageUnitTest {
+
+    private List<Values> values = Arrays.asList(
+            new Values(1, 10),
+            new Values(3, 20),
+            new Values(5, 30),
+            new Values(7, 50),
+            new Values(9, 40)
+    );
+
+    private Double expected = 6.2;
+
+    @Test
+    void twoPass() {
+        double top = values.stream()
+                .mapToDouble(v -> v.value * v.weight)
+                .sum();
+        double bottom = values.stream()
+                .mapToDouble(v -> v.weight)
+                .sum();
+
+        double result = top / bottom;
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void onePass() {
+        double top = 0;
+        double bottom = 0;
+
+        for (Values v : values) {
+            top += (v.value * v.weight);
+            bottom += v.weight;
+        }
+
+        double result = top / bottom;
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void expanding() {
+        double result = values.stream()
+                .flatMap(v -> Collections.nCopies(v.weight, v.value).stream())
+                .mapToInt(v -> v)
+                .average()
+                .getAsDouble();
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void reduce() {
+        class WeightedAverage {
+            final double top;
+            final double bottom;
+
+            public WeightedAverage(double top, double bottom) {
+                this.top = top;
+                this.bottom = bottom;
+            }
+
+            double average() {
+                return top / bottom;
+            }
+        }
+
+        double result = values.stream()
+                .reduce(new WeightedAverage(0, 0),
+                        (acc, next) -> new WeightedAverage(
+                                acc.top + (next.value * next.weight),
+                                acc.bottom + next.weight),
+                        (left, right) -> new WeightedAverage(
+                                left.top + right.top,
+                                left.bottom + right.bottom))
+                .average();
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void customCollector() {
+        class WeightedAverage implements Collector<Values, WeightedAverage.RunningTotals, Double> {
+            class RunningTotals {
+                double top;
+                double bottom;
+
+                public RunningTotals() {
+                    this.top = 0;
+                    this.bottom = 0;
+                }
+            }
+
+            @Override
+            public Supplier<RunningTotals> supplier() {
+                return RunningTotals::new;
+            }
+
+            @Override
+            public BiConsumer<RunningTotals, Values> accumulator() {
+                return (current, next) -> {
+                    current.top += (next.value * next.weight);
+                    current.bottom += next.weight;
+                };
+            }
+
+            @Override
+            public BinaryOperator<RunningTotals> combiner() {
+                return (left, right) -> {
+                    left.top += right.top;
+                    left.bottom += right.bottom;
+
+                    return left;
+                };
+            }
+
+            @Override
+            public Function<RunningTotals, Double> finisher() {
+                return rt -> rt.top / rt.bottom;
+            }
+
+            @Override
+            public Set<Characteristics> characteristics() {
+                return Collections.singleton(Characteristics.UNORDERED);
+            }
+        }
+
+        double result = values.stream()
+                .collect(new WeightedAverage());
+        assertEquals(expected, result);
+    }
+
+    private static class Values {
+        int value;
+        int weight;
+
+        public Values(int value, int weight) {
+            this.value = value;
+            this.weight = weight;
+        }
+    }
+}