I tried to complete a Java application that must include at a minimum: Three classes minimum…

I tried to complete a Java application that must include at a
minimum:

Three classes minimum
At least one class must use inheritance
At least one class must be abstract
JavaFX front end – as you will see, JavaFX will allow you to
create a GUI user interface.
The User Interface must respond to events.
If your application requires a data backend, you can choose to
use a database or to use text files.
Error handling – The application should be able to handle
invalid data.

There will be multiple assignments associated with this
application including:

UML – provide documentation that documents
your classes using UML.

Note: Since the user interface will use JavaFX, these classes
should be unrelated to the user interface of your application.
These classes should be related to the functionality of your
project. For example, let’s say your application will require the
user to enter information about a person (first name, last name,
etc.). You would want to create a class that contains all the
fields as well as the setters and getters required for the object
(with the setters doing validation as required for the associated
field). This class might also contain information about which
fields are required for the object to be valid as well as an
override of the toString method for the class to output all the
fields using the toString method in some predefined format. The
JavaFX user interface could then use the object when trying to
validate the user or to output the object to a report, etc. What
these classes should not contain is anything related to the user
interface for the application. That will come in the JavaFX portion
of the project.

Classes / Test Classes – you should have
created the classes your application will require at a minimum, in
addition to any “test” classes that will instantiate your
application classes and verify that these classes work. These
“test” classes should output to the console test messages that
verify your classes perform correctly.

Note: After this assignment is completed, you may find you need
some additional classes to complete your project. You may add
classes as required to make the app fully functional. However, make
sure you get approval for any changes you need to make and remember
to update your UML document as you will need to submit this
document along with the final project.

JavaFX – your non-GUI related
code should have been completed.as well as JavaFX GUI widgets
(controls / windows, etc.). You should be working on finalizing the
events and having a minimum of two events completed as well as your
complete GUI (i.e. Windows and fields).

Semester Project: Finalize Project –
including all requirements as outlined above and approved for this
project should have been met.
The following is what I created for the user to place their
order by entering their food and drink from my restaurant menu
The Model Classes are Food.java and
Drinks.java
The Abstract Class are Price.java
The Controller file are
FXMLDocumentController.java and FXML frontend FXML File and
FXMLDocument.fxml
The Main File is
myRestaurantFX.java
I am having trouble doing the error checking on my code based on
the answer from this link:
https://www.chegg.com/homework-help/questions-and-answers/need-java-application-gui-includes-following-requirements-three-classes-minimum-least-one–q40207800