Refactored ReverseStringUsingStack utility for reversing strings using stack

Author: navaneedan07

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

@@ -1,6 +1,8 @@
 package com.thealgorithms.strings;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.util.stream.Stream;
 import org.junit.jupiter.params.ParameterizedTest;
@@ -37,4 +39,16 @@ public void testReverseString3(String input, String expectedOutput) {
     public void testReverseString4(String input, String expectedOutput) {
         assertEquals(expectedOutput, ReverseString.reverse4(input));
     }
+
+    @Test
+    public void testReverseString4WithNull() {
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> {
+            ReverseString.reverse4(null);
+        });
+
+        String expectedMessage = "Input string cannot be null";
+        String actualMessage = exception.getMessage();
+
+        assertTrue(actualMessage.contains(expectedMessage));
+    }
 }