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COSC3311 – Software Design

COSC3311 – Software Design. Once Routines, Hockey Pucks & The Singleton Pattern. Instance Sharing – SOOSA p81-83. Very often groups of related objects share a common supplier a set of diskless workstations may all share a common file sharer In a hockey game, all players share the same puck.

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COSC3311 – Software Design

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  1. COSC3311 – Software Design Once Routines, Hockey Pucks &The Singleton Pattern Object Oriented Software Construction 22/03/2009 11:51 PM0

  2. Instance Sharing – SOOSA p81-83 • Very often groups of related objects share a common supplier • a set of diskless workstations may all share a common file sharer • In a hockey game, all players share the same puck Object Oriented Software Construction 22/03/2009 11:51 PM1

  3. Non-sharing version Object Oriented Software Construction 22/03/2009 11:51 PM2

  4. Sharing version Object Oriented Software Construction 22/03/2009 11:51 PM3

  5. Non-sharing class HOCKEY_PLAYER create make feature puck: PUCK name: STRING make (s:STRING) is -- hockey player do create puck.make(s) name := s io.put_string(puck.name) end end Object Oriented Software Construction 22/03/2009 11:51 PM4

  6. Sharing class HOCKEY_PLAYER2 create make feature puck: PUCK2 is -- shared puck once create Result.make(name) end name: STRING make (s:STRING) is -- hockey player do name := s io.put_string(puck.name) io.new_line end end Object Oriented Software Construction 22/03/2009 11:51 PM5

  7. Complete run-time state Object Oriented Software Construction 22/03/2009 11:51 PM6

  8. Patterns • Designing good OO software products is hard – you must find the right classes and establish key relationships between them at the right level of granularity. • Certain problems occur over and over again. • A design pattern describes a recurring problem and then describes a re-usable core solution. • Available in Steacie Library • Design Patterns, Gamma, Helm, Johnson and Vlissides, Addison Wesley, 1995. • (Eiffel) Design Patterns and Contracts http://www.irisa.fr/prive/jezequel/DesignPatterns/ Object Oriented Software Construction 22/03/2009 11:51 PM7

  9. Singleton Pattern (creational) • Problem – shared data • Solution – Ensure a class that encapsulates the shared data has only one instance and provide a global point of access to it. Object Oriented Software Construction 22/03/2009 11:51 PM8

  10. Banking system Object Oriented Software Construction 22/03/2009 11:51 PM9

  11. Need for shared data Shared data such as interests rates and minimum balance … Too many inheritance links?? Object Oriented Software Construction 22/03/2009 11:51 PM10

  12. Alternative – Singleton pattern Object Oriented Software Construction 22/03/2009 11:51 PM11

  13. SHARED_DATA class SHARED_DATA feature minimum_savings_balance: INTEGER set_minimum_savings_balance (b: INTEGER) is do minimum_savings_balance := b end prime_interest_rate: REAL is 2.0 feature {NONE} singleton: SHARED_DATA is once Result := Current end invariant only_one_instance: Current = singleton end Object Oriented Software Construction 22/03/2009 11:51 PM12

  14. ACCESS_DATA class ACCESS_DATA feature data: SHARED_DATA is -- access to unique instance of shared data once create Result end feature {NONE} -- invariant implementation is_real_singleton: BOOLEAN is -- multiple calls to `data' returns same instance do Result := data = data end invariant is_real_singleton end Object Oriented Software Construction 22/03/2009 11:51 PM13

  15. ACCOUNT class ACCOUNT create make feature data: SHARED_DATA make (initial_amount: INTEGER) is -- Set up account with `initial_amount'. local ad: ACCESS_DATA do create ad; data := ad.data create deposits.make; create withdrawals.make deposits.extend (create {DEPOSIT}.make (initial_amount)) end balance: INTEGER is do Result := deposits.total - withdrawals.total end minimum_balance: INTEGER is do Result := data.minimum_savings_balance end … end Object Oriented Software Construction 22/03/2009 11:51 PM14

  16. Unit Test test_account_deposit_and_withdraw: BOOLEAN is -- Use a bank account. local an_account: ACCOUNT data: SHARED_DATA access_data: ACCESS_DATA do comment("test_account_deposit_and_withdraw") create access_data data := access_data.data data.set_minimum_savings_balance (999) create an_account.make (2000) Result := an_account.minimum_balance = 999 check Result end an_account.deposit (500) if an_account.may_withdraw (1500) then an_account.withdraw (1500) end Result := an_account.balance = 1000 end Object Oriented Software Construction 22/03/2009 11:51 PM15

  17. Demo • Bank2.zip Object Oriented Software Construction 22/03/2009 11:51 PM16

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