refactor: Enhance docs, code, add tests in HarshadNumber (#6745)

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

@@ -1,49 +1,78 @@
 package com.thealgorithms.maths;
 
-// Wikipedia for Harshad Number : https://en.wikipedia.org/wiki/Harshad_number
-
+/**
+ * A Harshad number (or Niven number) in a given number base is an integer that
+ * is divisible by the sum of its digits.
+ * For example, 18 is a Harshad number because 18 is divisible by (1 + 8) = 9.
+ * The name "Harshad" comes from the Sanskrit words "harṣa" (joy) and "da"
+ * (give), meaning "joy-giver".
+ *
+ * @author <a href="https://github.com/Hardvan">Hardvan</a>
+ * @see <a href="https://en.wikipedia.org/wiki/Harshad_number">Harshad Number -
+ *      Wikipedia</a>
+ */
 public final class HarshadNumber {
     private HarshadNumber() {
     }
 
     /**
-     * A function to check if a number is Harshad number or not
+     * Checks if a number is a Harshad number.
+     * A Harshad number is a positive integer that is divisible by the sum of its
+     * digits.
      *
-     * @param n The number to be checked
-     * @return {@code true} if {@code a} is Harshad number, otherwise
+     * @param n the number to be checked (must be positive)
+     * @return {@code true} if {@code n} is a Harshad number, otherwise
      *         {@code false}
+     * @throws IllegalArgumentException if {@code n} is less than or equal to 0
      */
     public static boolean isHarshad(long n) {
         if (n <= 0) {
-            return false;
+            throw new IllegalArgumentException("Input must be a positive integer. Received: " + n);
         }
 
-        long t = n;
+        long temp = n;
         long sumOfDigits = 0;
-        while (t > 0) {
-            sumOfDigits += t % 10;
-            t /= 10;
+        while (temp > 0) {
+            sumOfDigits += temp % 10;
+            temp /= 10;
         }
 
         return n % sumOfDigits == 0;
     }
 
     /**
-     * A function to check if a number is Harshad number or not
+     * Checks if a number represented as a string is a Harshad number.
+     * A Harshad number is a positive integer that is divisible by the sum of its
+     * digits.
      *
-     * @param s The number in String to be checked
-     * @return {@code true} if {@code a} is Harshad number, otherwise
+     * @param s the string representation of the number to be checked
+     * @return {@code true} if the number is a Harshad number, otherwise
      *         {@code false}
+     * @throws IllegalArgumentException if {@code s} is null, empty, or represents a
+     *                                  non-positive integer
+     * @throws NumberFormatException    if {@code s} cannot be parsed as a long
      */
     public static boolean isHarshad(String s) {
-        final Long n = Long.valueOf(s);
+        if (s == null || s.isEmpty()) {
+            throw new IllegalArgumentException("Input string cannot be null or empty");
+        }
+
+        final long n;
+        try {
+            n = Long.parseLong(s);
+        } catch (NumberFormatException e) {
+            throw new IllegalArgumentException("Input string must be a valid integer: " + s, e);
+        }
+
         if (n <= 0) {
-            return false;
+            throw new IllegalArgumentException("Input must be a positive integer. Received: " + n);
         }
 
         int sumOfDigits = 0;
         for (char ch : s.toCharArray()) {
-            sumOfDigits += ch - '0';
+            if (Character.isDigit(ch)) {
+                sumOfDigits += ch - '0';
+            }
         }
 
         return n % sumOfDigits == 0;