Connie's Convenience Store - About Peter Coad

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34 Connie’s Convenience Store Establishing Responsibilities for Problem-Domain Objects model (and later, in code), a cashier object is an abstraction; it’s not the real thing; it’s a figment of your imagination. A cashier object “knows” and “does” things that the system needs to do, pertaining to the actual cashier. The cashier object and an actual cashier match up perfectly. The cashier object’s attributes and the actual cashier’s attributes match up, too. #50. “Select Attributes from Actual Ones” Strategy establishing responsibilities / what I know (fundamentals) • This is an aspect of a software object coming to life: “I know selective things that describe the actual object that I’m an abstraction of.” • Select the attributes for your software objects, taken from the abundance of the attributes that could be used to describe a corresponding actual (real-world) object. • Consider attributes from a broad-sweeping perspective; then work your way into the specific ones you need. Look at attributes in general, in this domain, and ultimately just within this system’s responsibilities. The cashier object’s services and the actual cashier’s services match up, too—but only if you are developing a simulation system. Otherwise, and that means for most applications, the services are remarkably different. Why? Because the services for an object in an object model are those services that it must fulfill to do its part in achieving the system’s responsibilities. And the services that an object in an object model is best suited for are the ones which take advantage of and fully utilize what that object knows and who that object knows. Then an object comes to life with what it does. You want to ask the question “Is authorized?” Which automated object should you turn to? Or you want to send the command “assess performance for a time interval.” Who does this work for you? The worker is a cashier object, of course. It’s the one with sufficient knowledge to coordinate getting these services accomplished. (Otherwise, without this approach, you end up with a model of data-holding “objects” and function-blob “objects.” Yuck!) By letting a cashier object take on these services itself, you get lower coupling, stronger cohesion, and domain-based partitioning of services (not just domain-based partitioning of attributes). What a deal! A side note: personification is a vital aspect of “object think.” It’s one of the most important principles on where to place a service. It helps object modelers strive for distributed attributes and services, based upon the domain-based partitioning of classes. It helps people let go of “DFD think” and “ERD think.” 2 Moreover, personification itself is extremely engaging. Try it out, especially when working out dynamics with scenarios. What kind of services do you need for cashier? 2 DFD stands for data flow diagram and ERD stands for entity-relationship diagram. These are both part of design methods that separate data and functions. Yuck!
Establishing Responsibilities for Problem-Domain Objects Connie’s Convenience Store 35 #90. “Service as a Question” Strategy establishing responsibilities / what I do (fundamentals) • Ask: what questions can an object answer? • Some good starting words to choose from: has, how many, how much, includes, is. • Why: stronger encapsulation, better partitioning of services; fewer objects that are mere data holders. One useful cashier service answers the question “Is authorized?” Write it down like this: – is authorized. What’s another? #91. “Service as a Verb” Strategy establishing responsibilities / what I do (fundamentals) • Some good service name verbs to choose from: activate (initialize, initiate, open, start) answer (reply, respond) assess (appraise, assay, evaluate, value) calculate (compute, count, estimate, rate, tally) deactivate (close, end, shut down, terminate) determine (decide, figure out, observe, resolve) find (get, look for, pinpoint) measure (bound, gauge, limit) monitor (conduct, direct, manage, observe, operate, supervise, watch) qualify (characterize, differentiate, discriminate, distinguish, mark) select (choose, cull, elect, opt for, pick). • Append “over interval” for services that apply to different time intervals. Another useful cashier service is: – assess performance. If a service is something that could be applied over many intervals of time, add “over interval” to the service name (Figure 1–23). Revised, a cashier’s services are: – is authorized – assess performance over interval.
Page 1 and 2: GETTING STARTED 1 Connie’s Conven
Page 3 and 4: Identifying System Purpose and Feat
Page 5 and 6: Selecting Objects Connie’s Conven
Page 7 and 8: Selecting Problem-Domain Objects Co
Page 19 and 20: Applying Patterns: Select and Organ
Page 29 and 30: Establishing Responsibilities for P
Page 33: Establishing Responsibilities for P
Page 55 and 56: Applying Patterns: Establish Proble
Page 61 and 62: Working Out Problem-Domain Dynamics
Page 63 and 64: Working Out Problem-Domain Dynamics
Page 65 and 66: Scenario: sale calculates its total
Page 67 and 68: Selecting Human-Interaction objects
Page 69 and 70: Establishing Human Interaction Resp
Page 71 and 72: Establishing Human Interaction Resp
Page 73 and 74: Working Out Human-Interaction Dynam
Page 75 and 76: Scenario: actual cashier logs on. S
Page 77 and 78: Scenario: sell item. Receipt SaleLi
Page 79 and 80: Scenario: get payment by check Rece
Page 81 and 82: Selecting System-Interaction Object
Page 83 and 84: Establishing System-Interaction Res
Page 85 and 86:
Working Out System-Interaction Dyna
Page 87 and 88:
Selecting Data-Management Objects a
Page 89 and 90:
Working Out Data-Management Dynamic
Page 91 and 92:
Progress at This Point Connie’s C
Page 93 and 94:
Progress at This Point Connie’s C
Page 95:
Wrap Up Connie’s Convenience Stor
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Connie's Convenience Store - About Peter Coad

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