Create packages, import packaged classes and interfaces into your programs, move packages, and encapsulate them in jar files.

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.

Java's default constructor lets developers create instances of classes when an explicitly defined constructor does not exist. Here's how it works.

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.