Still having trouble with ClassCastException?

ClassCastException is still the most common search term that sends people to this blog, although "swim across the atlantic ocean" is about to overtake it thanks, I can only guess, to fans of Detroit Roller Derby (don't ask ...).

So to be helpful to those readers who are not from Michigan I thought I'd clarify things as much as I can, be warned though it isn't much.

Because Sun's spec doesn't enforce a requirement for ClassCastExceptions to show the classes of the objects which you are trying to cast (see this earlier post) it is normally only possible to make any sense of a ClassCastException by reading the source code, and you really need to attach a debugger and examine the object that your programme is trying to cast before you can start to figure out why it won't work.

If your object seems to be of the correct class the problem is probably related to ClassLoaders, this article explains succinctly why that happens.

Sometimes, if you're smart, you can make a smart guess.

Sometimes (more often if programmers will take my advice) the exception message will tell you, but don't count on it.


If you are seeing a ClassCastException being thrown by a product you have installed then you need to check its documentation.

If the documentation doesn't help then you need to get help from the people who wrote it.