Object-oriented design CSE 432: Object-Oriented Software Engineering
Goals of OO analysis (quick review) n What are the two main goals of OO analysis? 1) Understand the customer’s requirements 2) Describe problem domain as a set of classes and relationships n What techniques have we studied for the 1 st goal? q q n Develop a requirements specification Describe scenarios of use in user’s language as use cases What techniques have we studied for the 2 nd goal? q q q CRC cards discover classes and run simulations UML class diagrams represent classes & relationships Sequence diagrams model dynamic behavior of a system
Goals of OO design n OO design develops the analysis into a blueprint of a solution q n Where does the “blueprint” metaphor come from? OO design starts by fleshing the class diagrams q Coad & Nicola call this "the continuum of representation principle: use a single underlying representation, from problem domain to OOA to OOD to OOP, " i. e. , class diagrams q Reworks and adds detail to class diagrams, e. g. , attribute types, visibility (public/private), additional constraints q Looks for opportunities for reuse q Addresses performance issues, both for system and users q Designs UI, database, networking, as needed q Designs ADT to describe the semantics of classes in more detail q Develops unit test plans based on class diagrams and ADT design
Four activities of design (Coad) n 1) Improve domain analysis: reuse and performance q q n OOA focuses primarily on the describing problem domain itself OOD reexamines the domain with an eye to practical concerns Reuse: factor out common code in abstract classes Performance tradeoffs: efficiency vs. effectiveness 2) Human interaction: encapsulates user interface q q HIC knows how to present data, not how to compute it Separation of concerns principle: Keep problem domain classes distinct from human interaction classes. Why? Loose coupling facilitates software reuse and maintenance An example: the Smalltalk Model-View-Controller framework: n n n q model is the problem domain view is the human interface (windows that view the problem domain objects) controller is the mouse or keyboard input, also interacting with P. D. objects C++ Interviews is two part framework: subject (problem domain) & views (UI)
How does this design illustrate the separation of concerns principle?
Four activities of design (continued) n 3) Task management q q q n Multi‑tasking and concurrency considerations How does separation of concerns apply here? Examples? 4) Data management q q Storage and retrieval of external data Database design (relational or object-oriented) Or database interface (JDBC, CORBA) How does separation of concerns apply here?
Reworking the class diagram n Add new relations implied in classes q q q Inverse operations (e. g. , undo/redo, get. Data/set. Data) Factor complex behaviors out as classes themselves If only difference between subclasses is presentation of data, use a service n n n Should we derive Hex. Count, Binary. Count, Oct. Count from Count? Rather, add a service, as. Base, to present data in different bases Distinguish client (has-a) and inheritance (is-a) relations q q Misuse of multiple inheritance is when a derived class is "composed of" several parent classes I. e. , class AIRPLANE has parents WINGS, ENGINE, TAIL) But the behavior of AIRPLANE is not just the sum of its parts Bjarne Stroustrup's heuristic: "can it have two? " n Then it's a containment, or has-a relation
Reworking the class diagram (continued) n Heuristic: use inheritance to extend existing classes q q n Generalize common behaviors in abstract classes q n E. g. , Complex. Matrix adapts Array or Ordered. Cltn But avoid adapting parents for the sake of theirs (open‑closed principle) E. g. , Mouse, Tablet and Keyboard can all inherit behavior from an abstract class, Input. Device Use multiple inheritance for compound classes q q q Teaching. Assistant inherits from both Instructor and Student Window as a compound of Rectangle (graphical behaviors) and Tree (hierarchical behaviors) Note: many OO languages don’t support multiple inh.
Classes as active state machines n Consider whether a class should keep track of its own internal state q Example from Bertrand Meyer: first cut design of LINKED_LIST class LINKABLE[T] ‑‑linkable cells feature value: T; right: LINKABLE[T]; ‑‑next cell ‑‑routines to change_value, change_right end; class LINKEDLIST[T] feature nb_elements: INTEGER; first_element: LINKABLE[T]; value(i: INTEGER): T is ‑‑value of i‑th element; loop until it reaches the ith element insert(i: INTEGER; val: T); ‑‑loop until it reaches ith element, then insert val delete(i: INTEGER); ‑‑loop until it reaches ith element, then delete it n n n Problems with first‑cut? Getting the loops right is tricky (loops are error‑prone) Redundancy: the same loop logic recurs in all these routines q q Reuse leads to inefficiency: suppose I want a routine search Find an element then replace it: I'll do the loop twice! Need some way to keep track of the position I found! Could return the LINKABLE cell found, but this would ruin encapsulation
Classes as active state machines (cont. ) n Instead, view LINKED_LIST as a machine with an internal state q n Internal state is information stored as attributes of an object What have we added to represent internal state? q q q q Cursor: current position in the list search(item) routine moves the cursor until it finds item insert and delete operate on the element pointed at by cursor How does this simplify the code of insert, delete, etc. ? Client has a new view of LINKED_LIST objects: l. search(item); ‑‑find item in l if not offright then delete end; ‑‑delete LINKABLE at cursor Other routines move cursor: l. back; l. forth
Key idea for OOD: data structures can be active q q n Active structures have internal states, which change Routines manipulate the object's state What other classes could be designed this way? q q q Files, random number generators, tokenizers, . . . Class as state machine view may not be obvious during analysis A good reason for redesign!
Case Study: Full‑Screen Entry Systems n Straightforward data processing application: menu‑driven data entry (see overhead) q q q n Each menu comes with a panel of information & lets user choose next action Interaction during a airline reservation session Enquiry on flights, information & possible new states Meyer shows different ways to solve problem q q q goto flow (50's), functional decomposition (70's) OO design (90's): improves reusability and extensibility
State diagrams in UML n States in ovals, transitions as arrows q q n Transitions labels have three optional parts: Event [Guard] / Action Item Received is an event, /get first item is an action, [Not all items checked] is a guard State may also label activities, e. g. , do/check item q q Actions, associated with transitions, occur quickly and aren’t interruptible Activities, associated with states, can take longer and are interruptible n Definition of “quickly” depends on the kind of system, e. g. , real-time vs. info system
Super-states n Example shows a super-state of three states q q n Concurrency in state diagrams q q n Can draw a single transition to and from a super-state How does this notation make things a bit clearer? Dashed line indicates that an order is in two different states, e. g. Checking & Authorizing When order leaves concurrent states, it’s in a single state: Canceled, Delivered or Rejected When should you create UML state diagrams?
UML package diagrams n UML packages for higher level structure (Coad’s subjects) n Denoted by box with smaller box on top q q n Packages can be grouped in higher-order packages q q q n Note dependency arrows A dependency indicates that changes to one element may cause changes to the other Packages may include packages Common package as <<global>> means all packages in system have dependency to this one General package marked {abstract} means this package is an interface, with subtypes Heuristic: divide classes into packages; analyze dependencies; refactor to reduce dependencies
Design process procedures n Conduct peer reviews at every stage of process q q q n CRC cards: run simulations to discover missing responsibilities Conduct peer review of class diagrams: does it cover the domain? Explain design, looking for opportunities for iterative redesign Contract design: peer review ADTs or assertions Test design: review plans for unit and acceptance testing Plan testing on a per class basis q q q Write system and unit test plan descriptions as part of class design Unit test all public member functions Test for valid, invalid and boundary cases System testing follows thorough class testing See Junit tool for automated test generation support
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