This call to a generic collection method contains an argument with an incompatible class from that of the collection's parameter (i.e., the type of the argument is neither a supertype nor a subtype of the corresponding generic type argument). Therefore, it is unlikely that the collection contains any objects that are equal to the method argument used here. Most likely, the wrong value is being passed to the method.

In general, instances of two unrelated classes are not equal. For example, if the Foo and Bar classes are not related by subtyping, then an instance of Foo should not be equal to an instance of Bar. Among other issues, doing so will likely result in an equals method that is not symmetrical. For example, if you define the Foo class so that a Foo can be equal to a String, your equals method isn't symmetrical since a String can only be equal to a String.

In rare cases, people do define nonsymmetrical equals methods and still manage to make their code work. Although none of the APIs document or guarantee it, it is typically the case that if you check if a Collection<String> contains a Foo, the equals method of argument (e.g., the equals method of the Foo class) used to perform the equality checks.