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Object Behavior Analysis: some guidelines

Object Behavior Analysis: some guidelines. Methods should be distributed fairly evenly among classes, so that you avoid having a few “control” objects and many “data” objects When possible, assign method to class that knows the attributes needed to perform the method

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Object Behavior Analysis: some guidelines

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  1. Object Behavior Analysis: some guidelines • Methods should be distributed fairly evenly among classes, so that you avoid having a few “control” objects and many “data” objects • When possible, assign method to class that knows the attributes needed to perform the method • when no single class knows all the attributes needed, assign method to the class that would be logically expected to perform the method

  2. Guidelines [contd] • Methods used to create and break relationships should be assigned to the class that controls the relationship • Method that changes the state of an object should be assigned to the class. • When in doubt, act it out!

  3. Example Rental Customer Authorized User Rental customer can exist without be associated with Authorized User, but not vice-versa. So, method AddAuthorizedUser() should be assigned to Rental Customer (the controlling class) Example Two classes: Account and Payment. A method reportStatus(..) returns values “overdue” or “paid”. Which class should contain this method?

  4. Example Method addAccount() creates a relationship between a Customer instance and an Account instance. Each customer can be related to zero or many Accounts objects, but each Account object must be related to exactly one Customer object.Which class should this method be assigned to? Example A method calcExtendedPrice(..) multiplies the quantity of a line item by the price of the line item. Quantity is an attribute of LINE-ITEM and price is an attribute PRODUCT. Should this method be assigned to LINE-ITEM or PRODUCT?

  5. Evaluating Object-behavior design • Present functionality vs. future adaptability (reusability) • Coupling: • should not have to pass a lot of data between objects - no message should need more than three parameters • No messages should be sent from sub-classes to access data in a super-class - taken care of by inheritance. • Coupling is a natural side-effect of specialization and composition relationships - however, collaboration relationships should not be so strong that Class-A and useless without Class-B.

  6. Evaluating Object-behavior design [contd.] • Cohesiveness • single functionality per method Eg. completeOrderFooter(..) that also prints the order - two separate methods better • overriding of attributes and methods defined in a super-class should not be too frequent. • Clarity • Complexity vs. reusability • neither too deep, nor too wide

  7. Evaluating Object-behavior design [contd.] • Simplicity • should not have too many methods in a class • 5 or 6; number of attributes less than twice number of methods • too many methods signify an application specific definition of a class, which will be difficult to reuse. • “Act-ability” • high quality OO design can be acted out be humans playing the role of objects • can be verified by a structured walk-through • mappings between real-world objects and system objects should be transparent.

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