Link to pistoneventpost
Fires after the piston has moved and set surrounding states. This will not fire if PistonPreEvent is cancelled.
Importing the class
Link to importing-the-class
It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.
Link to extending-pistonevent
PistonEventPost extends PistonEvent. That means all methods available in PistonEvent are also available in PistonEventPost