Which exception are you getting? Is it a ClassCastException?
The bit about $EntrySetEntry means there is an inner or nested class inside AbstractAttributeMap called EntrySetEntry. Most Map implementations have EntrySet inner or nested classes. I think nested, but I am not sure.
You cannot cast to "some other object." You can only cast an object reference to a class which the object is already an instance of. So if you are casting ((Foo) myObjectReference.)bar() it means that myObjectReference must already be an instance of Foo (and Foo must have a method called bar()). If whatever you are casting doesn't refer to an instance of that class, the JVM will be unable to execute the cast and you get the ClassCastException.
But you haven't actually told us which Exception you are getting.