Attempts to cancel execution of this task. This attempt will fail if the task has already completed, has already been cancelled, or could not be cancelled for some other reason. If successful, and this task has not started when {@code cancel} is called, this task should never run. If the task has already started, then the {@code mayInterruptIfRunning} parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.
Returns {@code true} if this task was cancelled before it completed normally.
Returns {@code true} if this task completed.