public void save(Object obj) { Assert.isNull(obj, "Object must be null"); // save object to database }In this example, the `save` method checks that the `obj` parameter is `null` before saving it to the database. If it is not `null`, an `IllegalArgumentException` with the message "Object must be null" will be thrown. This `Assert` class is part of the Spring Framework, and can be found in the `spring-core` package library.