@Test public void testNull() { String nullString = null; assertNull(nullString); }
@Test public void testNotNull() { String notNullString = "Hello world!"; assertNull(notNullString); }In this example, `notNullString` contains a value, so the `assertNull` method should fail, and the test should fail. The `assertNull` method belongs to the `junit.framework` package, which is part of the JUnit library.