add test case 3

Author: dbc2201

.idea/workspace.xml

@@ -7,6 +7,7 @@
 import org.junit.jupiter.params.provider.ValueSource;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 class DuplicateCharactersCounterTest {
     private DuplicateCharactersCounter counter;
@@ -39,4 +40,10 @@ void shouldReturnCount1ForNonDuplicateCharacters() {
         // Assert
         assertEquals(1, count, "the count value for all non duplicate characters should be 1");
     }
+
+    @Test
+    @DisplayName("should throw an IllegalArgumentException for null input")
+    void shouldThrowAnIllegalArgumentExceptionForNullInput() {
+        assertThrows(IllegalArgumentException.class, () -> counter.countDuplicateCharacters(null));
+    }
 }