Abstract Class Is Not Part Of Hierarchy

All other classes are non abstract.

Abstract class is not part of hierarchy. Consider the following class hierarchy. Note that code reuse is a very compelling reason to use abstract classes as long as the is a relationship within the class hierarchy is preserved. However since identityuser is not an abstract class it can in theory be instantiated and assigned too and the mysql database provider requires a registration for it too even if in production there will never be an value with this discriminator. Abstract classes are simply that.

Now we need to represent a class for anything that can be displayed. And java 8 adds another wrinkle with default methods which can sometimes take the place of needing to create an abstract class altogether. Shape is an abstract class declaration provided in subsequent section. Forming the hierarchy eliminates redundant code and organizes our abstractions into something easier to understand.

Displayitem is an abstract class designed to be subclassed to be useful. Abstract classes are useful when creating hierarchies of classes that model reality because they make it possible to specify an invariant level of functionality in some methods but leave the implementation of other methods until a specific implementation of that class a derived class is needed. Well anticipate the need for many different types of things that are to be displayed and make this class the top level of this particular part of the class hierarchy. Abstract means of organizing shared information.

The isa hierarchy is a compile time hierarchy in that it involves classes extending other classes. An example main is provided that processes all of the shapes in the form of the menu.