OOPS Object-Oriented Programming in Smalltalk

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.

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

Practical Session 2

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 (e.g. Pie1 Walk) All objects in the same class have the same attributes and respond in the same way to each message.

Representing Objects This is an object diagram for an object from the PieEater Class It shows: class attributes messages that can be understood Class of Object: PieEater row 2 column 5 direction ”SE” colour ClrRed pen True Protocol: walk, turnLeft, turnRight, home, green, red, penDown, penUp, inspect

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

Practical Session 2 This session introduces you to a new class of object- PastieEater This has a different start position and colour

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.

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.

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

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.

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”

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 object’s response,

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.

SameColourAs collaboration sameColourAs: Pie2 colour ClrGreen colour: ClrGreen User Pie1Pie2