Refactored ReverseStringUsingStack utility for reversing strings using stack

Author: navaneedan07

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

@@ -39,6 +39,7 @@ void testWhitespaceHandling() {
 
     @Test
     void testNullInput() {
-        assertThrows(IllegalArgumentException.class, () -> { ReverseStringUsingStack.reverse(null); });
+        IllegalArgumentException thrown = assertThrows(IllegalArgumentException.class, () -> { ReverseStringUsingStack.reverse(null); });
+        assertEquals("Input string cannot be null", thrown.getMessage());
     }
 }

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

@@ -41,14 +41,9 @@ public void testReverseString4(String input, String expectedOutput) {
     }
 
     @Test
-    public void testReverseString4WithNull() {
-        Exception exception = assertThrows(IllegalArgumentException.class, () -> {
-            ReverseString.reverse4(null);
-        });
-
+    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));
     }
 }