Object-Oriented Design

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Identifying Objects

• Identify the nouns (objects) in the problem description.
• Identify more objects in your developing problem solution.
• From these, determine what new types (classes) you need.
  Example: BlackJack Program
  • cards
  • a deck
  • a dealer
  • player(s)

  • Do different cards represent different types? Or are suit and rank attributes of a single card type?
  • Are the dealer and player instances of the same class? I.e., objects of the same type?
  • Should there be a separate Hand class?

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Identifying Operations

• Identify what can be done to objects.     or
• Identify what objects can do for themselves.     or
• Identify who is responsible for doing what.
• Terminology: operations/responsibilities/contracts/message passing
  Example: BlackJack Program
  • Card: display, report value (report suit? maybe internally) (modify value/suit? or set these in constructor only?)
  • Deck: shuffle, deal next card, report how many cards left in deck
  • Dealer: deal cards, add card to hand, show (sometimes partial) hand, calculate (partial) hand value, number of cards in hand, hit?, determine winner, reset hand
  • Player: add card to hand, show hand, calculate hand value, number of cards in hand, hit?, reset hand

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Identifying Relationships

• Identify object relationships, e.g. what objects need to know about what other objects, what objects are part of other objects.
• Terminology: encapsulation; aggregation/has-a relationships; knows-about relationships
  (avoid is-a relationships: inheritance!)
  Example: BlackJack Program
  • Card:
  • Deck: has cards
  • Dealer: has deck, has cards (or hand, which has cards), knows about player(s)
  • Player: has cards (or has hand, which has cards)

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Scenarios

• Develop scenarios of how the objects interact to see if you have identified the right objects, the right operations, and the right relationships.
• Develop common scenarios (this is how the thing should usually work).
• Remember to also develop uncommon scenarios, "weird cases," boundary conditions.
• Design is incremental and iterative!   (Grady Booch)

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Implementation

• Design the public interface (public member function declarations).
• Design the internal data representation (private data members). Include information about relationships.
• Implement the public member functions. Identify useful internal helper functions (private member functions).
• Test individual objects and member functions before putting everything together.

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Alyce Brady, Kalamazoo College