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*MET12-EX0:* This guidelinerule may be violated provided that the incompatible types are never compared.  There are classes in the Java platform libraries (and elsewhere) that extend an instantiable class by adding a value component.  For example, {{java.sql.Timestamp}} extends {{java.util.Date}} and adds a nanoseconds field. The {{equals}} implementation for {{Timestamp}} violates symmetry and can cause erratic behavior if {{Timestamp}} and {{Date}} objects are used in the same collection or are otherwise intermixed \[[Bloch 2008|AA. Bibliography#Bloch 08]\].