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Today I found a very very weird behavior of the JVM, a (normally type-safe) List<Date> did actually hold a List<MyObject> at runtime! I wonder how that ever could happen, but couldn't find anything in the web. That's the situation: I'm working with Spring Data JPA 1.2.0 on a JBoss EAP 6.0 Server with JRE 1.6.0_18-b07 . By mistake, in a Spring Data JPA Repository class there was written a wrong result type in the @Query expression. It should have been: @Query("select distinct trunc(record.orderDateTime) from MyType record [...]" ) public List<Date> getOrderDates(...); But was: @Query("select record from MyType record [...]" ) public List<Date> getOrderDates(...); So, the aim was to load a list of dates ( java.util.Date ), which works fine if the query is properly defined as in the first code snippet. But that coding mistake led to the following result: At runtime, there actually WAS a List<MyType> returned, even though the method's signature defines a List<Date> . Also in my model an attribute of List<Date> was/contained a List<MyType> . I debugged it and couldn't believe my eyes! I could even write the content of this list to a JSP (I only recognized this weird behavior because a JSP couldn't be displayed any more due to a Spring Expression Language error occuring with the attempt to type match from MyType to Date , which of course had to crash). Hell, shall I now loose my believe in the type-safety of Java? Has anybody ever had such a problem? Does an explanation for this exist? And can I do anything to fix that or is it a general issue? Maybe another version of the JRE, JBOSS, ...?

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