CSC 335 ObjectOriented Programming and Design

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CSC 335 Object-Oriented Programming and Design

• A Few More Design Considerations

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Some Object-Oriented Design Guidelines

• Uses Relationships between objects
• Expert
• Low Coupling
• High Cohesion
• Polymorphism
• Dont Talk to Strangers

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Relationship Between Objects

• There are relationships between objects
• uses relationship
• containment relationship
• There are relationships between classes
• inheritance relationship
• Let's consider how objects use other objects

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Uses Relationship

• A uses relationship occurs when one object sends
  another a message. Example
• A Person object sends a setAlarm message to an
  AlarmClock object
• public void prepareToSleep( )
• Time wakeupTime new Time(7, 15)
• theAlarmClock.setAlarm(wakeupTime)

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Implementing the Uses Relationship

• The sender must know the receiver's name
• An object's instance variable is first and
  foremost a uses relationship
• Every object "knows" the receiver's name
• a JukeboxAccountCollection has a Vector of
  BankAccounts
• The collection can send the messages to the Vector

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Cars and Gas Stations

• 1 If a Car "contains" a gas station instance
  variable
• send the give_me_gas message to its gas station
• If a Car does not "contain" a gas station
• how can a Car send a give_me_gas message?
• 2 A Car could be given the name of the gas
  station as a parameter
• // aDispatcher object sends this message
• carj.getGasoline(Exxon126)

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Other ways to implement the uses relationship

• 3 Let the car find a gas station on its own
• Assume map is a global variable everyone knows
• myGasStation map.gasStationLocation()
• myGasStation.give_me_gasoline(10.00)
• 4 Whenever a car needs gas, let the car build it
• It will know the name of the object it creates
• The gas station is destroyed upon exit

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Referential attribute (not nice)

• 5 Tell the car who its gas station is when the
  car is built (referential attribute)
• provides access to an entire object that was
  constructed elsewhere

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The Uses Relationship

• The uses relationship is typical among the top
  level objects in an application
• your Collection contains a Vector
• Pass PlayerList as an argument
• Have a global object like System.out
• Have a method construct objects
• Store a referential attribute
• Get a reference to an object through a message

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Design GuidelineRiel's Heuristic 4.1

• Minimize the number of objects with which another
  collaborates
• Worst case design has a collection of simple
  classes where all use each other.
• Could one class wrap several smaller classes?
• Example on next slide

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A Meal class that wraps up three smaller classes
Melon
cost()
Restaurant Patron
cost()
Steak
Meal
Pie
cost()
Melon
cost()
Restaurant Patron
cost()
cost()
Steak
Pie
cost()
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Meal Wrapper Class

• Now it looks like there are 4 messages instead of
  3
• encapsulation makes it look look 1 however
• Containment simplifies things
• Restaurant Patron collaborates with one object
  now, instead of three

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Design Guideline

• Containment implies a uses relationship, so
  messages should be sent to contained objects
• If no messages are sent to a contained object, it
  doesn't belong. Put it in the right class
• Exception Container classes
• List, Vector, HashTables, etc. do not send
  messages to the objects
• their job is adding, finding, and removing
  elements

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Riel's Heuristic 4.6 and 4.7

• Most of the methods in a class should be using
  most of the instance variables most of the time.
• If not, perhaps we should have two classes
• Classes should not contain more objects than a
  developer can fit in his/her short term memory
• 7 plus or minus 2 (5 to 9 instance variables max)

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Heuristic 4.13

• A class must know what it contains, but the
  con-tained class should not know who contains it.
• It's okay for a bedroom too know that it has a
  clock, but the clock shouldn't be dependent on
  the bedroom
• an account should not send messages to DB
• We can reuse account in other places

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Heuristic 4.14

• Objects that share lexical scope--those contained
  in the same containment--should not have uses
  relationships between them.
• Consider an ATM which contains a card reader and
  a display screen
• The card reader should not send a message to the
  display screen
• better reuse--could use the card reader at a door
  without taking along the display screen

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Violation of 4.14 also increases
complexity

• An ATM may contain
• card reader, display screen, and keypad
• The ATM should coordinate activities between
  these three objects
• We dont need to have additional collaborations

Automated Teller Machine
HaveACard?()
displayPIN()
Display
Card Reader
getPIN()
Keypad
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4.11 Containment by Value Versus Containment by
Reference

• Containment by value
• one object contains another object
• the contained object dies when the container dies
• Containment by reference allows for
• sharing of an object by a group of objects