Visual Basic: An Object Oriented Approach

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Visual Basic An Object Oriented Approach
2 Designing Software Systems
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Class and Object

• A Class is a template for an Object
• Develop a class get many objects
• A Class defines how an object is encapsulated
• Some parts of an object are not accessible by a
  user of the object (Private)
• Other parts define the objects interface with
  its user (Public)
• In general, object data is private, and the
  operations that are provided to work with it are
  public
• The attributes or properties of an object reflect
  its internal data, and are public operations for
  accessing it

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Class/Object Diagrams

• We can depict a class by describing its interface
• We can depict an object by stating its class and
  showing its property values

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Object Relationships

• There are a number of ways that objects can
  interact in a program
• Inheritance a Class relationship
• Composition a Class relationship
• Aggregation a Class relationship
• Collaboration (message passing) an Object
  relationship
• Class relationships are defined for the whole
  class
• Object relationships occur between specific
  objects, or instances of a class

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Inheritance

• One class of objects can be created as a
  specialist version of another class
• e.g. a scientific calculator is a specialist type
  of calculator
• There are generally considered to be two types of
  inheritance
• Code inheritance an existing class is reused as
  the basis for a new class
• Interface inheritance a new class conforms to
  the same interface as an existing class
• Visual Basic only implements the latter
• The next version of Visual Basic will implement
  Code Inheritance

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Composition

• An object can be made up of several other objects
• e.g. an email message has a recipient address, a
  subject and a body
• A powerful tool for creating complex classes
• Top-down decomposition/bottom-up composition

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Aggregation

• An object can contain multiples of other objects
• e.g. a spreadsheet has a number of cells
• Aggregation is another form of composition
• Only difference is that multiplicity is provided
  for
• On the diagram, can use for many objects, a
  specific number, or allowable range (e.g. 1..10)

spreadsheet

cell
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Collaboration (message passing)

• An object can invoke the services of another
• e.g. a document can send data to a printer
• A message is how one object gets another to do
  something for it
• Message can include information to be passed to
  the serving object
• Message format also allows for information to be
  returned from the serving object

a document
print (Hello World)
System Printer
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Classes

• Candidates for a class
• Are things that are named in the system
  description at the requirements stage
• Can not normally be defined as a simple value
  (e.g. colour, length, number of wheels)
• These are attributes, also known as Properties
• Will normally have certain behaviours, operations
  or actions associated with them
• Can support interactions with other things
• Are able to alter their state in response to
  messages

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Designing Classes and Interactions

• Start by defining how a user will interact with a
  class or set of classes
• A Use-Case diagram shows this
• Indicates user and interactions with the system
  (possibly one class) as a whole

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Define a class

• From the use-case diagram
• Identify class(es) required
• Decide on Properties of class
• Decide on Methods of class

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Properties and Methods

• A Property of a class
• is some attribute that each object in the class
  has its own copy of
• can take on a number of possible values
• can be complex - a Property can be an object
• can be defined in such a way that it can be
• retrieved, or
• changed, or
• both in response to a message
• A Method
• is an operation that any object of class can
  perform
• can require extra information to do its job
• can return a result to the object that invoked it
• is able to alter the internal data of an object
  of the class

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Messages

• A Message is a request to invoke a Method or
  access a Property of an object
• normally will come from another object
• can be in response to an event that has occurred
  e.g. the user has clicked a mouse button
• A message describes the entire interaction
• the object that sent it
• the object that is to receive it
• information passed along with it (e.g. a new
  Property value)
• any result that it has (e.g. the result of a
  calculation performed by the receiving object)

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Object Interactions

• The ATMMachine object sends these messages
• Deposit must include the amount to be deposited
• Withdraw must include the amount to be
  withdrawn
• Balance will return a result the account
  balance
• The AnAccount object (a member of the Account
  class), is the recipient of the messages

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Summary

• A Class is a template for one or more objects
• an object is an instance of class
• Classes and Objects relate to other classes or
  Objects in various ways
• Classes define (and Objects have)
• Properties
• Methods
• An interaction between objects is done by one
  object passing a message to another