Abstract classes and interfaces in Java serve fundamentally different purposes. Learn the differences between these Java language elements and how to use them in your programs.

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

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.

An object that implements the Comparable interface must call the "implements" keyword first, and then the "compareTo" method must be defined in the Comparable class.