OOPS

1
OOPS

• Object-Oriented Programming in Smalltalk

2
Practical Sessions 1-5

• These sessions introduce you to the Smalltalk
  Environment, and use the familiar idea of
  PieEater to examine a number of important
  concepts.
• The Environments are there for you to explore,
  and to ensure that you have understood the basic
  ideas.
• The most important of these ideas are summarised
  in this lecture.

3
Practical Session 1

• Practical Session 1 introduces you to a new
  environment, and some new PieEaters.

4
Practical Session 2
5
Practical Session 1

• What you should Learn
• Each of the PieEaters is an Object, to which we
  can send messages.
• We can inspect the state of each object
• Messages to objects maybe via buttons or Text,
  and take the form
• Receiver ltMessagegt (e.g. Pie1 Walk)
• All objects in the same class have the same
  attributes and respond in the same way to each
  message.

6
Representing Objects
 

• This is an object diagram for an object from the
  PieEater Class
• It shows
• class
• attributes
• messages that can be understood

 
7
Protocol

• The Protocol of an object is simply the set of
  messages that an object can understand, together
  with the effects of those messages

• message
• home
• left

• effect
• receiver moves to position 1
• reduces position value by 1

8
Practical Session 2

• This session introduces you to a new class of
  object- PastieEater
• This has a different start position and colour

9
Practical Session 2
10
Practical Session 2

• What you should learn
• Objects belonging to different classes will have
  some attributes and/or protocols different.
• Sometimes classes can share some of the same
  attributes and protocols. This is called
  Polymorphism.

11
Practical Session 3

• This introduces you to a new object called
  FlashPieEater
• This object can respond to all the messages that
  PieEater can respond to, but also has one further
  attribute and additional protocols.

12
Practical Session 3
13
Practical Session 3

• What you should learn
• Subclasses of objects are objects which have all
  the attributes of the original class
  (superclass), but have additional attributes (and
  protocol)
• We can construct Class Hierarchy Diagrams

14
Practical Session 4

• Session 4 introduces the idea of message
  arguments.
• This is achieved in this case by a colour ...
  widget
• widget device
• Note that green and red are commands which have
  been programmed but there are no commands to
  change the colour to blue, for example.

15
Practical Session 4
16
Practical Session 4

• What you should learn
• Messages to objects may sometimes require an
  argument to be given.
• For example the message
• Pie5 colour ClrBlack
• will send a message to Pie5 to change its colour
  to black.
• These messages can be programmed into widgets

17
Practical Session 5

• In this session we look at different ways of
  communicating with objects.
• There is a new widget
• and a message reply box, for the objects
  response,

18
Practical Session 5
19
Practical Session 5

• What you should learn
• All messages to objects return information
  (whether you want it or not)
• The message usually is a description of the state
  of the object after it has completed the action.
• We can interrogate objects as to their state, and
  we can use this idea to get objects to
  collaborate, by asking one to interrogate
  another, and act on the outcome.

20
SameColourAs collaboration
Pie1
Pie2
User

• sameColourAs Pie2

• colour
• ClrGreen
• colour ClrGreen