By Rick Mercer with help from

1
The Relationships Among Objects

• By Rick Mercer with help from
• Object-Oriented Design Heuristics
• Arthur Riel
• Addison-Wesley, 1996, ISBN 0-201-6338-X

2
Relationship Between Objects

• Object relationships occur when one object
• sends a message to an instance variable
• is passed as an argument in a message
• creates another object
• reference is returned from a message
• stores a referential attribute

3
Example Cars and Gas Stations

• 1 If a Car "contains" a gas station as an
  instance variable (not intuitive)
• A car can send a getGasoline message to its own
  gas station
• myGasStation.getGasoline(10.00)
• 2 If a Car does not "contain" a gas station, a
  Car could be given the name of the gas station as
  an argument
• // aDispatcher object sends this message
• aCar.getGasoline(aGasStation)

4
Other uses relationships

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

5
Who constructs needed objects?

• What object creates object X? ignoring
  special-cases designs such as Factory
• Choose an object B to create an X such that
• B contains X
• B closely uses X
• B has the initializing data for X

6
Referential attribute (can be bad)

• 6 Store a reference to a gas station that was
  created elsewhere
• Known to some as a referential attribute
• This provides access to an entire object that was
  constructed elsewhere
• The car has a reference needed by someone else
• Be careful This can lead to incorrect behavior
• Would be better to have just one instance with a
  global point of reference use Singleton to be
  discussed later