Objects by Sufian Idris

©Sufian Idris, Jabatan Sains KomputerUniversiti Kebangsaan Malaysia

Objects


• Object is a very important concept in object-orientation.

• When an OO program is executed, it normally creates objects in memory.

• These objects collaborate with each other to perform certain tasks.

• This realizes the runtime behaviour of the program.


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Object Space


Definition of Object

• An object is a thing that has state, behaviour and identity.

• Examples:– The LCD projector in the lecture hall at

FTSM.– Dr Sufian’s Kenari– Dr Sufian’s Waja


Object State

• Each object has attributes which collectively represent its state.

BEFORE AFTER


• In Java, the attributes of an object are implemented as variables that belong to that object. These variables are called instance variables.

Example:

Car

“WHH7801”

green

“Sufian Idris”

colour

plateNr

owner

Car

“WHY9531”

blue

“Ali Bakar”

colour

plateNr

owner

A Car object Another Car object


• The atributes of an object normally do not change.

The values of the attributes of an object can change. However, there are attributes with fixed values.

Fridge

“342-0967”

“Ali Bakar”

biru

owner

serialNr

colour

value is fixed

values can be changed


• An object may have links to other objects. These links represent relationships between those objects.

• Example:

: Car

: Person

: Carlinks

The links are added to represent the fact that both cars are owned by the same person.


• Example:

: Car

: Engine

: Chassis

links

The links are added to represent the fact that the engine and the chassis are part of the car. This relationship is also called aggregation.


In Java, links to objects are represented as object references.

Instance variables can be defined to store such references.

: Computer

datePurch

processor

memory

15032002

: Memory

size 256

: Processor

speed 900

object references


Object Behaviour

• Message sending is a mechanism which objects use to interact with other objects.

• To request an object to perform a certain task, an appropriate message needs to be sent to that object.

jump


An object only responds to messages that it understands.

fly??


Object behaviour refers to how an object reacts to messages it receives from its clients.

: Scanner: Temperature

Converter

nextFloat()

The Scanner object responds byinputting a float value and returningthat value to the TemperatureConverterobject.


: Rectangle

5

colour

width

height 10

green

• An object’s response to a message can cause its state to change.

zoom_2x

10

20


An object may also send messages to other objects in its response to a message.

switchChannel(5) openCommChannel()switchChannel(5)closeCommChannel()


Object Identity

Each object has its own identity which differentiates it from other objects.

Triplets… but each of them has its own identity


The identity of an object is built-in. There is no need to explicitly define an instance variable to represent the identity of an object.

class Computer {private int computerID;private Date datePurchased;private Processor processor;

…}

Redundant


Relationship between Object State and Object Behaviour

• Object behaviour can affect object state.• Example:

An ATM machine dispensing money causes the amount of money in the machine to decrease.


Object state can affect object behaviour.

Number of tins:

20

What if there are no more tins in the machine?

dispense()


Abstraction

• Supposing we need to define an object that represents a cat. What attributes and methods should we define?

Attributes Behaviour

weightlengthheart size

chewdrinkrun

… …

…there are too many!


Humans have the ability to isolate those aspects that are important for some purpose and suppress those aspects that are unimportant.


A program designer only needs to define the attributes and behaviour that are essential to the problem in hand.

In object-orientation, software objects are abstractions.

: Bird

5

colour

weight

height 10

white

14

0.5


Encapsulation

• Objects should not reveal all of its information. There is certain information that should be hidden.

• Example A restaurant may not want to reveal the specifics of how the dishes it serves are prepared. On the other hand, it needs to reveal the price of each type of dish.


• Encapsulation is the technique for packaging information in such a way as to hide what should be hidden, and make visible what is intended to be visible.


Information normally hidden by objects:object attributesthe specifics of how they react to the

messages received by them

40

area()

attributes:widthheight…

behaviour:

area()

return width * height;

HIDDEN BY OBJECT


Some benefits of encapsulation:if done properly, modifications to

object implementation should not affect clients

objects can control access to their attributes


Object Interface

Not all messages are understood by an object.

To obtain information on the messages which an object understands, we need to refer to its interface.


The interface of an object reveals public information about the object. It should not contain information hidden by the object.

An example of information that could be in an object’s interface is a list of its public methods; only their signature, not their implementation.


INTERFACEarea()setWidth(int width)setHeight(int height)

??

yesss!

But where can we find information about an object’s interface?

Rectangleobject


Messages and Methods

• Message sending is a mechanism used by objects to communicate with other objects.

• To get an object to do something, you send an appropriate message to that object.

jump


A message consists of its name (compulsory) and its parameters (if any).

The receiver object is the object receiving the message. The sender of the message is referred to as the client.

switchChannel(5) openCommChannel()switchChannel(5)closeCommChannel()

clientreceiverobject


An object implements its behaviour with methods. A method contains implementation details of how an object responds to a certain message.

40

area()

attributes:widthheight…

behaviour:

area()

return width * height;

message

method


Object Collaboration

• Isolated objects are useless in object-oriented systems.

“No object is an island”


Object cooperation is very important in object-oriented software. In general, the behaviour of an OO software is realized through object collaboration.

Objects by Sufian Idris

Education

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