Doug Lea writes:

Problem: ScheduledThreadPoolExecutor.remove can throw exception when it should
return false.

Symptoms: Calling ScheduledThreadPoolExecutor.remove with a task type
that was not produced by a ScheduledThreadPoolExecutor schedule method
should simply return false. However, it may instead throw a
ClassCastException while traversing for a non-existent task.

Solution: Add a checked cast before trying to remove.