Refactored ReverseStringUsingStack utility for reversing strings using stack

Author: navaneedan07

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

@@ -11,15 +11,18 @@ private ReverseStringUsingStack() {
      * @return reversed string
      */
     public static String reverse(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
         if (str.isEmpty()) {
             return str;
         }
         // Push each character of the string onto the stack
-        for (char i : str.toCharArray()) {
-            stack.push(i);
+        for (char ch : str.toCharArray()) {
+            stack.push(ch);
         }
         // Pop each character from the stack and append to the StringBuilder
         while (!stack.isEmpty()) {

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

@@ -1,5 +1,7 @@
 package com.thealgorithms.strings;
 
+import java.util.Stack;
+
 /**
  * Reverse String using different version
  */
@@ -57,4 +59,30 @@ 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
+        if (str.isEmpty()) {
+            return str;
+        }
+        // Push each character of the string onto the stack
+        for (char ch : str.toCharArray()) {
+            stack.push(ch);
+        }
+        // Pop each character from the stack and append to the StringBuilder
+        while (!stack.isEmpty()) {
+            reversedString.append(stack.pop());
+        }
+        return reversedString.toString();
+    }
 }

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); });
     }
 }

src/test/java/com/thealgorithms/strings/ReverseStringTest.java

@@ -31,4 +31,10 @@ public void testReverseString2(String input, String expectedOutput) {
     public void testReverseString3(String input, String expectedOutput) {
         assertEquals(expectedOutput, ReverseString.reverse3(input));
     }
+
+    @ParameterizedTest
+    @MethodSource("testCases")
+    public void testReverseString4(String input, String expectedOutput) {
+        assertEquals(expectedOutput, ReverseString.reverse4(input));
+    }
 }