Type: Code Smell Detection
By contract, the equals(Object) method, from java.lang.Object, should accept a null argument. Among all the other cases, the null case is even explicitly detailed in the Object.equals(...) Javadoc, stating "For any non-null reference value x, x.equals(null) should return false."
Assuming that the argument to equals is always non-null, and enforcing that assumption with an annotation is not only a fundamental violation of the contract of equals, but it is also likely to cause problems in the future as the use of the class evolves over time.
The rule raises an issue when the equals method is overridden and its parameter annotated with any kind of @Nonnull annotation.