Introduction to Classes and OO Programming.

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Introduction to Classes and OO Programming.
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• One major problem with the work covered so far is
  that we have created programs based solely on a
  single form.
• When programming real systems in VB.NET your
  project is seldom limited to a single form.
• Many programs have a number of forms (components)
  and these forms need to be managed in a sensible
  way.

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Hire Car System.

• May involve the following forms
• Password Screen.
• Main Menu.
• Customer Details Form.
• Booking Form.

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Issues to Consider with Multiple Forms.

• 1. Problems with code duplication.
• We dont want sections of code repeated.
• 2. Problems with code re-use.
• We want to be able to recycle our code.

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Consider A Search Facility.

• Imagine you had a form in your system which
  allows you to search for cars, e.g. those that
  are free for hire on a specific date, those that
  are of a certain make etc. This form is used to
  make bookings for customers.

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Search Facility Version 1

• Search button on a form.
• Code exists within a Sub Procedure within the
  form.

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Search Facility Version 2

• Search for cars that are in need of
  servicing/various states of damage.

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Code Duplication.

• The first problem we have created here is
  potential for code duplication.
• By having a search facility in both forms we have
  two sections of search code which are going to be
  pretty similar.

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A Solution?

• One solution might be to only place the search
  code behind one of the forms and then somehow
  call the search from one form to another.
• Q. Is this an acceptable solution?
• We have solved the problem of duplication but are
  now faced with a few other problems.
• Firstly to call the code in one form from another
  we need to have both forms open, when all that we
  want is the facility to search.

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Q. So how do we solve this problem?

• A. We introduce the idea of a Class Module.

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Features of a Class.

• Two main features of a class are methods and
  properties.
• Methods (Sometimes referred to as Operations.)
  e.g. DoSearch.
• Properties (Sometimes referred to as Attributes)
  e.g. Count.

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Methods and Properties You have Used Already!

• Methods.
• lstStudents.Items.Add
• lstStudents.Items.RemoveAt
• MessageBox.Show("Hello World")
• Properties.
• txtInput.Text
• btnDelete.Enabled

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The Reminder Application.

• For the next four weeks we shall be building a
  small but useable multi form application called
  Reminders.
• The program allows the user to set timed
  reminders (alarms) which will "go off" at
  specified points in time.
• In a moment we shall look at the finished program.

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The Universal Modelling Language Documentation.

• In addition to the program you have been provided
  with a set of documentation using the Universal
  Modelling Language, (UML).
• UML is a set of documents that may be used to
  plan design and document object oriented (OO)
  programs like the Reminder application.
• Some of you have seen UML documentation in your
  Systems Analysis and Design Module.

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Instantiation Classes v Objects.

• When you create a class, you are creating a
  template from which you make your objects.
• For example in Word you have a set of templates
  for such things as letters and memos. When you
  create a new letter, you take a copy of the
  template and make an actual instance of a letter.

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Encapsulation Public v Private.

• One important feature of using classes is that we
  really don't care how they work.
• For example to add an item to a list box we might
  use the following line of code
• lstMyList.Items.Add("Hello")
• The Add method is all we are interested in, we
  don't care how it adds the word "Hello" to the
  list just that it will do it.

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Project Description GNiteall plc

• GNiteall is a company specialising in the
  manufacture of aromatic oils to assist with
  insomnia. Unfortunately due to the effectiveness
  of their products and continued exposure, the
  employees of the company are often drifting into
  a soporific state and falling asleep.
• The management wish to address this problem by
  ensuring that each employee has installed on
  their computer a reminder system which will alert
  staff of important appointments (which they are
  often missing).
• The initial stages of the project will allow the
  user to add, edit and delete timed reminders that
  will activate at the appropriate time and alert
  the user. A later revision of the program will
  add sound thus waking the user, but at this stage
  a basic implementation is proposed.

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Use Case Diagram.
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Use Case Description.

• Reminder Management
• The user adds a new reminder to the reminder
  list.
• The user edits an existing reminder in the
  reminder list.
• The user deletes a reminder in the reminder list.
• The user opens a reminder list.
• The user closes a reminder list.
• The user snoozes a reminder by 10 minutes.
• The timer triggers an active reminder.

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Class Diagram.