You must create a timer class instance to use the timer's functions. Add the following code to the Java file:timer = new Timer (countdown, this);Notice the "countdown" variable is used.

Sealed classes in Java let developers limit the creation and use of subclasses and preserve the class hierarchy. Here is how sealed classes in Java work.

This post explains how to use classes in Java to build objects and organize your code. Also learn: constructors, static methods, and more!