Add different types of Mean (#4339)

src/main/java/com/thealgorithms/maths/Means.java

@@ -0,0 +1,54 @@
+package com.thealgorithms.maths;
+
+import java.util.stream.StreamSupport;
+import org.apache.commons.collections4.IterableUtils;
+
+/**
+ * https://en.wikipedia.org/wiki/Mean
+ * <p>
+ * by: Punit Patel
+ */
+public final class Means {
+
+    private Means() {
+    }
+
+    /**
+     * @brief computes the [Arithmetic Mean](https://en.wikipedia.org/wiki/Arithmetic_mean) of the input
+     * @param numbers the input numbers
+     * @throws IllegalArgumentException empty input
+     * @return the arithmetic mean of the input numbers
+     */
+    public static Double arithmetic(final Iterable<Double> numbers) {
+        checkIfNotEmpty(numbers);
+        return StreamSupport.stream(numbers.spliterator(), false).reduce((x, y) -> x + y).get() / IterableUtils.size(numbers);
+    }
+
+    /**
+     * @brief computes the [Geometric Mean](https://en.wikipedia.org/wiki/Geometric_mean) of the input
+     * @param numbers the input numbers
+     * @throws IllegalArgumentException empty input
+     * @return the geometric mean of the input numbers
+     */
+    public static Double geometric(final Iterable<Double> numbers) {
+        checkIfNotEmpty(numbers);
+        return Math.pow(StreamSupport.stream(numbers.spliterator(), false).reduce((x, y) -> x * y).get(), 1d / IterableUtils.size(numbers));
+    }
+
+    /**
+     * @brief computes the [Harmonic Mean](https://en.wikipedia.org/wiki/Harmonic_mean) of the input
+     * @param numbers the input numbers
+     * @throws IllegalArgumentException empty input
+     * @return the harmonic mean of the input numbers
+     */
+    public static Double harmonic(final Iterable<Double> numbers) {
+        checkIfNotEmpty(numbers);
+        return IterableUtils.size(numbers) / StreamSupport.stream(numbers.spliterator(), false).reduce(0d, (x, y) -> x + 1d / y);
+    }
+
+    private static void checkIfNotEmpty(final Iterable<Double> numbers) {
+        if (!numbers.iterator().hasNext()) {
+            throw new IllegalArgumentException("Emtpy list given for Mean computation.");
+        }
+    }
+}