Fix checkstyle violations and add explicit cast

Author: duvvuvenkataramana

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

@@ -1,9 +1,9 @@
 package com.thealgorithms.maths;
 
 /**
- * Armstrong Number (also known as Narcissistic Number)
- * A number is called an Armstrong number if the sum of cubes of its digits equals the number itself.
- * For example: 153 = 1³ + 5³ + 3³ = 1 + 125 + 27 = 153
+ * 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
  */
@@ -12,23 +12,23 @@ private ArmstrongNumber() {
     }
 
     /**
-     * Check if a number is an Armstrong number
+     * Checks if a number is an Armstrong number.
      *
      * @param number the number to check
-     * @return true if the number is an Armstrong number, false otherwise
+     * @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;
-        int digits = String.valueOf(number).length();
 
         while (number > 0) {
             int digit = number % 10;
-	    sum += (int) Math.pow(digit, digits);
+            sum += (int) Math.pow(digit, numDigits);
             number /= 10;
         }
 

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

@@ -6,32 +6,44 @@
 import org.junit.jupiter.api.Test;
 
 class ArmstrongNumberTest {
+    @Test
+    void testSingleDigitNumbers() {
+        for (int i = 0; i <= 9; i++) {
+            assertTrue(ArmstrongNumber.isArmstrong(i));
+        }
+    }
 
     @Test
-    void testArmstrongNumbers() {
-        // Test known Armstrong numbers
-        assertTrue(ArmstrongNumber.isArmstrong(0));
-        assertTrue(ArmstrongNumber.isArmstrong(1));
+    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() {
-        // Test numbers that are not Armstrong numbers
         assertFalse(ArmstrongNumber.isArmstrong(10));
         assertFalse(ArmstrongNumber.isArmstrong(100));
-        assertFalse(ArmstrongNumber.isArmstrong(152));
-        assertFalse(ArmstrongNumber.isArmstrong(200));
+        assertFalse(ArmstrongNumber.isArmstrong(123));
+        assertFalse(ArmstrongNumber.isArmstrong(999));
     }
 
     @Test
     void testNegativeNumbers() {
-        // Negative numbers cannot be Armstrong numbers
         assertFalse(ArmstrongNumber.isArmstrong(-1));
         assertFalse(ArmstrongNumber.isArmstrong(-153));
     }
+
+    @Test
+    void testZero() {
+        assertTrue(ArmstrongNumber.isArmstrong(0));
+    }
 }