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.
Noncompliant code example
public boolean equals(@javax.annotation.Nonnull Object obj) { // Noncompliant
// ...
}
Compliant solution
public boolean equals(Object obj) {
if (obj == null) {
return false;
}
// ...
}
