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Object design-part-2
- 1. Dhaval Dalal software-artisan.com Part II: Moving Towards a Simple Design OBJECT DESIGN
- 3. WAYS TO FULFILL DEPENDENCIES Create the collaborator within a constructor/ method. Perform a look-up for a collaborator. Pass the collaborator from outside. using a Constructor. using a Setter.
- 4. 4 INJECT DEPENDENCIES First of all, avoid concrete instantiation of collaborator objects within a constructor/method of an object. Avoid making call to ‘new’ If you still have to do it, then encapsulate call to ‘new’ by using A static Factory Method Factory. Builder.
- 5. 5 INJECT DEPENDENCIES/ CLOSURES Second option, do a “look-up” for the collaborator object Use a Service Locator. Third option, inject concrete implementation of a collaborator in to an object at run-time. Use a DI Container. Fourth option, inject code block instead of a collaborator.
- 6. 6 A GENERAL GUIDELINE Short Answer Dependency Elimination is better than Dependency Inversion Long Answer Favor Closure Injection (where its possible and sensible) over Dependency Injection over Look-up over Concrete Instantiation.
- 7. SETTER OR CONSTRUCTOR INJECTION? A tell tale sign of exposing internal implementation manifests as bunch of getters and setters on the object. Setters and Getters break the Command/Query Pattern (Tell, Don’t ask principle) Inject collaborators in Constructors.
- 8. THE SIDE EFFECT? Too many parameters in the Constructor!
- 9. <<STEREOTYPICAL RELATIONS>> OBJECT AND ITS COLLABORATORS Real Dependency Policy Part Notifications
- 10. GUIDELINES OF THUMB Pass only Real Dependencies through constructors. Without which an object cannot fulfill its responsibilities. For Policies and Parts Provide Defaults and allow them to be set through Setters. For Notifications Provide NULL/EMPTY Listeners as default and allow new ones to be added/removed using “Adders/Removers”
- 11. I STILL HAVE A BIG PARAMETER LIST? Too many things going on or have more than one concept in that object (SRP violation). Break-up the object. This will collapse the parameter list.
- 12. 12 THE RESULT? Fewer unintended consequences from code changes and more flexibility in your systems. Creates Objects that are isolated from each other. Makes large scale re-structuring of the system possible.
- 13. 13 VALUE OBJECTS Must have value semantics. Immutability Value once set, cannot be changed. Operations on them, returns new value objects Object’s Identity is its value Equality Strive to reuse value objects
- 14. REFERENCES Agile Principles, Patterns and Practices Robert C. Martin and Micah Martin Object Design Rebecca Wirfs-Brock and Alan McKean Alec Sharp The Pragmatic Programmers Mock Roles, not Objects Paper by Steve Freeman, Nat Pryce,Tim Mackinnon, Joe Walnes InfoQ presentation by Steve Freeman and Nat Pryce Head-First Design Patterns Book Venkat Subramaniam’s Blog