The Foundation of Game Play Loop: The JavaFX Pulse System and the Game Processing Architecture - Beginning Java 8 Games Development - page 227

NetBeans will create a bootstrap infrastructure for the GamePlayLoop.java class,
with a package and a class declaration, as illustrated in Figure 7-4 . Now, you add an
extends keyword and AnimationTimer.
Figure 7-4 . NetBeans creates a GamePlayLoop.java class and opens it in an editing tab in the IDE, for you to edit
Mouse over the error, press Alt+Enter , and select Add import , as displayed in
Figure 7-5 .