Added quadratic mean (#7315)

* Added quadratic mean

Added a quadratic mean of the given numbers, sqrt ((n1^2+n2^2+...+nk^2)/k).

* Added tests for quadratic mean

* Corrected quadratic mean

* Added comment to quadratic mean

* Corrected quadratic mean tests

* Replaced sqrt by pow

* Error fixed

* Extra whitespace removed

* Extra whitespace removed

* Removed extra white space

* Removed extra white space

Author: kvadrik

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

@@ -107,6 +107,28 @@ public static Double harmonic(final Iterable<Double> numbers) {
         return size / sumOfReciprocals;
     }
 
+    /**
+     * Computes the quadratic mean (root mean square) of the given numbers.
+     * <p>
+     * The quadratic mean is calculated as: √[(x₁^2 × x₂^2 × ... × xₙ^2)/n]
+     * </p>
+     * <p>
+     * Example: For numbers [1, 7], the quadratic mean is √[(1^2+7^2)/2] = √25 = 5.0
+     * </p>
+     *
+     * @param numbers the input numbers (must not be empty)
+     * @return the quadratic mean of the input numbers
+     * @throws IllegalArgumentException if the input is empty
+     * @see <a href="https://en.wikipedia.org/wiki/Root_mean_square">Quadratic
+     *      Mean</a>
+     */
+    public static Double quadratic(final Iterable<Double> numbers) {
+        checkIfNotEmpty(numbers);
+        double sumOfSquares = StreamSupport.stream(numbers.spliterator(), false).reduce(0d, (x, y) -> x + y * y);
+        int size = IterableUtils.size(numbers);
+        return Math.pow(sumOfSquares / size, 0.5);
+    }
+
     /**
      * Validates that the input iterable is not empty.
      *

src/test/java/com/thealgorithms/maths/MeansTest.java

@@ -172,6 +172,53 @@ void testHarmonicMeanWithLinkedList() {
         assertEquals(expected, Means.harmonic(numbers), EPSILON);
     }
 
+    // ========== Quadratic Mean Tests ==========
+
+    @Test
+    void testQuadraticMeanThrowsExceptionForEmptyList() {
+        List<Double> numbers = new ArrayList<>();
+        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, () -> Means.quadratic(numbers));
+        assertTrue(exception.getMessage().contains("Empty list"));
+    }
+
+    @Test
+    void testQuadraticMeanSingleNumber() {
+        LinkedHashSet<Double> numbers = new LinkedHashSet<>(Arrays.asList(2.5));
+        assertEquals(2.5, Means.quadratic(numbers), EPSILON);
+    }
+
+    @Test
+    void testQuadraticMeanTwoNumbers() {
+        List<Double> numbers = Arrays.asList(1.0, 7.0);
+        assertEquals(5.0, Means.quadratic(numbers), EPSILON);
+    }
+
+    @Test
+    void testQuadraticMeanMultipleNumbers() {
+        Vector<Double> numbers = new Vector<>(Arrays.asList(1.0, 2.5, 3.0, 7.5, 10.0));
+        double expected = Math.sqrt(34.5);
+        assertEquals(expected, Means.quadratic(numbers), EPSILON);
+    }
+
+    @Test
+    void testQuadraticMeanThreeNumbers() {
+        List<Double> numbers = Arrays.asList(3.0, 6.0, 9.0);
+        double expected = Math.sqrt(42.0);
+        assertEquals(expected, Means.quadratic(numbers), EPSILON);
+    }
+
+    @Test
+    void testQuadraticMeanIdenticalNumbers() {
+        List<Double> numbers = Arrays.asList(5.0, 5.0, 5.0);
+        assertEquals(5.0, Means.quadratic(numbers), EPSILON);
+    }
+
+    @Test
+    void testQuadraticMeanWithLinkedList() {
+        LinkedList<Double> numbers = new LinkedList<>(Arrays.asList(1.0, 5.0, 11.0));
+        assertEquals(7.0, Means.quadratic(numbers), EPSILON);
+    }
+
     // ========== Additional Edge Case Tests ==========
 
     @Test
@@ -198,21 +245,25 @@ void testAllMeansConsistencyForIdenticalValues() {
         double arithmetic = Means.arithmetic(numbers);
         double geometric = Means.geometric(numbers);
         double harmonic = Means.harmonic(numbers);
+        double quadratic = Means.quadratic(numbers);
 
         assertEquals(7.5, arithmetic, EPSILON);
         assertEquals(7.5, geometric, EPSILON);
         assertEquals(7.5, harmonic, EPSILON);
+        assertEquals(7.5, quadratic, EPSILON);
     }
 
     @Test
     void testMeansRelationship() {
-        // For positive numbers, harmonic mean ≤ geometric mean ≤ arithmetic mean
+        // For positive numbers, harmonic mean ≤ geometric mean ≤ arithmetic mean ≤ quadratic mean
         List<Double> numbers = Arrays.asList(2.0, 4.0, 8.0);
         double arithmetic = Means.arithmetic(numbers);
         double geometric = Means.geometric(numbers);
         double harmonic = Means.harmonic(numbers);
+        double quadratic = Means.quadratic(numbers);
 
         assertTrue(harmonic <= geometric, "Harmonic mean should be ≤ geometric mean");
         assertTrue(geometric <= arithmetic, "Geometric mean should be ≤ arithmetic mean");
+        assertTrue(arithmetic <= quadratic, "Arithmetic mean should be ≤ quadratic mean");
     }
 }