Certain code you override or implement is expected to adhere to contracts laid out in the Java SDK. Examples of these are:
A project called junit-addons on SourceForge has test superclasses that enable compliance testing of objects to these java contracts
Although these test helper classes work without issue, they are designed for pre-generic java, and JUnit 3.x and could be confusing to developers used to JUnit 4.x annotations style of test implementation.
An modernized JUnit4 version of these test helpers, with examples, is at https://github.com/chas66/TestHelpers