Fix armstrong number implementation

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

@@ -0,0 +1,37 @@
+package com.thealgorithms.maths;
+
+/**
+ * An Armstrong number is a number that is equal to the sum of its own digits
+ * each raised to the power of the number of digits.
+ * For example: 153 = 1^3 + 5^3 + 3^3 = 1 + 125 + 27 = 153
+ *
+ * @author duvvuvenkataramana
+ */
+public final class ArmstrongNumber {
+    private ArmstrongNumber() {
+    }
+
+    /**
+     * Checks if a number is an Armstrong number.
+     *
+     * @param number the number to check
+     * @return {@code true} if Armstrong number, {@code false} otherwise
+     */
+    public static boolean isArmstrong(int number) {
+        if (number < 0) {
+            return false;
+        }
+
+        int original = number;
+        int numDigits = String.valueOf(number).length();
+        int sum = 0;
+
+        while (number > 0) {
+            int digit = number % 10;
+            sum += (int) Math.pow(digit, numDigits);
+            number /= 10;
+        }
+
+        return sum == original;
+    }
+}

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

@@ -0,0 +1,49 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+class ArmstrongNumberTest {
+    @Test
+    void testSingleDigitNumbers() {
+        for (int i = 0; i <= 9; i++) {
+            assertTrue(ArmstrongNumber.isArmstrong(i));
+        }
+    }
+
+    @Test
+    void testThreeDigitArmstrongNumbers() {
+        assertTrue(ArmstrongNumber.isArmstrong(153));
+        assertTrue(ArmstrongNumber.isArmstrong(370));
+        assertTrue(ArmstrongNumber.isArmstrong(371));
+        assertTrue(ArmstrongNumber.isArmstrong(407));
+    }
+
+    @Test
+    void testFourDigitArmstrongNumbers() {
+        assertTrue(ArmstrongNumber.isArmstrong(1634));
+        assertTrue(ArmstrongNumber.isArmstrong(8208));
+        assertTrue(ArmstrongNumber.isArmstrong(9474));
+    }
+
+    @Test
+    void testNonArmstrongNumbers() {
+        assertFalse(ArmstrongNumber.isArmstrong(10));
+        assertFalse(ArmstrongNumber.isArmstrong(100));
+        assertFalse(ArmstrongNumber.isArmstrong(123));
+        assertFalse(ArmstrongNumber.isArmstrong(999));
+    }
+
+    @Test
+    void testNegativeNumbers() {
+        assertFalse(ArmstrongNumber.isArmstrong(-1));
+        assertFalse(ArmstrongNumber.isArmstrong(-153));
+    }
+
+    @Test
+    void testZero() {
+        assertTrue(ArmstrongNumber.isArmstrong(0));
+    }
+}