Link to fireball
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-abstracthurtingprojectile
Link to implemented-interfaces
Fireball implements the following interfaces. That means all methods defined in these interfaces are also available in Fireball