Refactored ReverseStringUsingStack utility for reversing strings using stack

Author: navaneedan07

src/main/java/com/thealgorithms/stacks/ReverseStringUsingStack.java

@@ -11,6 +11,10 @@ private ReverseStringUsingStack() {
      * @return reversed string
      */
     public static String reverse(String str) {
+        // Check if the input string is null
+        if (str == null) {
+            throw new IllegalArgumentException("Input string cannot be null");
+        }
         Stack<Character> stack = new Stack<>();
         StringBuilder reversedString = new StringBuilder();
         // Check if the input string is empty

src/main/java/com/thealgorithms/strings/ReverseString.java

@@ -59,12 +59,16 @@ public static String reverse3(String string) {
         }
         return sb.toString();
     }
+
     /**
      * Reverse version 4 the given string using a Stack.
      * This method pushes each character of the string onto a stack
      * and then pops them off to create the reversed string.
      */
     public static String reverse4(String str) {
+        if (str == null) {
+            throw new IllegalArgumentException("Input string cannot be null");
+        }
         Stack<Character> stack = new Stack<>();
         StringBuilder reversedString = new StringBuilder();
         // Check if the input string is empty

src/test/java/com/thealgorithms/stacks/ReverseStringUsingStackTest.java

@@ -39,6 +39,6 @@ void testWhitespaceHandling() {
 
     @Test
     void testNullInput() {
-        assertThrows(NullPointerException.class, () -> { ReverseStringUsingStack.reverse(null); });
+        assertThrows(IllegalArgumentException.class, () -> { ReverseStringUsingStack.reverse(null); });
     }
 }