The library includes numerous examples of static member classes, some of which are found in the Java 2D geometric shape classes located in the java.awt.geom package.

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.

While there are workarounds -- for example, you could introduce a Factory class to generate transaction objects -- this adds complexity and could become a maintenance burden. Another approach could be ...