Added program to check Abundant number

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

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+package com.thealgorithms.maths;
+
+/**
+ * In number theory, an abundant number or excessive number is a positive integer for which
+ * the sum of its proper divisors is greater than the number.
+ * Equivalently, it is a number for which the sum of proper divisors (or aliquot sum) is greater than n.
+ *
+ * The integer 12 is the first abundant number. Its proper divisors are 1, 2, 3, 4 and 6 for a total of 16.
+ *
+ * Wiki: https://en.wikipedia.org/wiki/Abundant_number
+ */
+public final class AbundantNumber {
+
+    private AbundantNumber() {
+    }
+
+    // Function to calculate sum of all divisors including n
+    private static int sumOfDivisors(int n) {
+        int sum = 1 + n; // 1 and n are always divisors
+        for (int i = 2; i <= n / 2; i++) {
+            if (n % i == 0) {
+                sum += i; // adding divisor to sum
+            }
+        }
+        return sum;
+    }
+
+    // Common validation method
+    private static void validatePositiveNumber(int number) {
+        if (number <= 0) {
+            throw new IllegalArgumentException("Number must be positive.");
+        }
+    }
+
+    /**
+     * Check if {@code number} is an Abundant number or not by checking sum of divisors > 2n
+     *
+     * @param number the number
+     * @return {@code true} if {@code number} is an Abundant number, otherwise false
+     */
+    public static boolean isAbundant(int number) {
+        validatePositiveNumber(number);
+
+        return sumOfDivisors(number) > 2 * number;
+    }
+
+    /**
+     * Check if {@code number} is an Abundant number or not by checking Aliquot Sum > n
+     *
+     * @param number the number
+     * @return {@code true} if {@code number} is a Abundant number, otherwise false
+     */
+    public static boolean isAbundantNumber(int number) {
+        validatePositiveNumber(number);
+
+        return AliquotSum.getAliquotSum(number) > number;
+    }
+}

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

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+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+class AbundantNumberTest {
+    @ParameterizedTest
+    @CsvSource({"12", "66", "222", "444", "888", "2424"})
+    void abundantNumbersTest(int n) {
+        assertTrue(AbundantNumber.isAbundant(n));
+        assertTrue(AbundantNumber.isAbundantNumber(n));
+    }
+
+    @ParameterizedTest
+    @CsvSource({"1", "2", "6", "111", "333", "2222"})
+    void nonAbundantNumbersTest(int n) {
+        assertFalse(AbundantNumber.isAbundant(n));
+        assertFalse(AbundantNumber.isAbundantNumber(n));
+    }
+
+    @ParameterizedTest
+    @CsvSource({"0", "-1"})
+    void throwsNegativeNumbersNotAllowed(int n) {
+        assertThrows(IllegalArgumentException.class, () -> AbundantNumber.isAbundant(n));
+        assertThrows(IllegalArgumentException.class, () -> AbundantNumber.isAbundantNumber(n));
+    }
+}