What is the reason for the failure of ImmutableCollection.contains(null)?

edit

The reason for the failure of ImmutableCollection.contains(null) is because null is not a valid value in some collections. Immutable collections are designed to be free of null values, and therefore do not support null as a valid value to search for. Additionally, some collections, such as TreeSet and TreeMap, have a natural ordering that cannot include null as it is not comparable to other values. It is generally recommended to avoid using null values in collections to avoid these issues.