refactor ShuffleArray: improve documentation and maintainability (#6357)

refactor ShuffleArray: Improve Documentation and Code Quality

src/main/java/com/thealgorithms/misc/ShuffleArray.java

@@ -17,19 +17,37 @@ import java.util.Random;
  * @author Rashi Dashore (https://github.com/rashi07dashore)
  */
 public final class ShuffleArray {
-    // Prevent instantiation
+
     private ShuffleArray() {
     }
 
     /**
-     * This method shuffles an array using the Fisher-Yates algorithm.
+     * Shuffles the provided array in-place using the Fisher–Yates algorithm.
      *
-     * @param arr is the input array to be shuffled
+     * @param arr the array to shuffle; must not be {@code null}
+     * @throws IllegalArgumentException if the input array is {@code null}
      */
     public static void shuffle(int[] arr) {
+        if (arr == null) {
+            throw new IllegalArgumentException("Input array must not be null");
+        }
+
         Random random = new Random();
         for (int i = arr.length - 1; i > 0; i--) {
             int j = random.nextInt(i + 1);
+            swap(arr, i, j);
+        }
+    }
+
+    /**
+     * Swaps two elements in an array.
+     *
+     * @param arr the array
+     * @param i   index of first element
+     * @param j   index of second element
+     */
+    private static void swap(int[] arr, int i, int j) {
+        if (i != j) {
             int temp = arr[i];
             arr[i] = arr[j];
             arr[j] = temp;