Link to pistonpreevent
Fires before the piston has updated block states.
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
PistonPreEvent extends PistonEvent. That means all methods available in PistonEvent are also available in PistonPreEvent