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.

Learn how to use synchronous and asynchronous callbacks in Java—including callbacks with lambda expressions, CompletableFuture, and more.

Learn how to use the Command pattern to encapsulate requests as objects that you can execute alone or in sequence in your Java programs.