Object-Oriented Design גרא וייס המחלקה למדעי המחשב אוניברסיטת בן-גוריון

Transcription

1 Object-Oriented Design גרא וייס המחלקה למדעי המחשב אוניברסיטת בן-גוריון

2 2 Why Start with Design? Object-oriented thinking begins with objectoriented design It is the easiest way to see the problems of realistic modern software development Without understanding programming in the large, one cannot appreciate the importance of OOP

3 3 Programming in the Small and Programming in the Large Programming in the small One programmer understands everything from top to bottom. Main problem is the development of algorithms Programming in the large System is developed by a large teams of programmers Main problems are management of details and communication between software subsystems

4 4 Basis for Design Consider for the moment what aspects of a problem are known first: Data Structures Functions A Formal Specification Behavior A design technique based on behavior can be applied from the very beginning of a problem Whereas techniques based on more structural properties necessarily require more preliminary analysis

5 5 Responsibility Driven Design A design technique that has the following properties: Can deal with ambiguous and incomplete specifications Naturally flows from Analysis to Solution Easily integrates with various aspects of software development

6 6 Directed Evolution Let us take specifications as they occur in nature Imprecise Ambiguous Unclear and rather than attempt to change human nature, let us direct the evolution of the specification in concert with the design of the software system

7 7 Evolution of Specifications

8 8 An Example, the IIKH Imagine you are the chief software architect in a major computing firm The president of the firm rushes into your office with a specification for the next PC-based product It is drawn on the back of a dinner napkin. Briefly, the Intelligent Interactive Kitchen Helper will replace the box of index cards of recipes in the average kitchen

9 9 Your Job Your job is to develop the software that will implement the IIKH:

10 10 Abilities of the IIKH Here are some of the things a user can do with the IIKH: Browse a database of recipes Add a new recipe to the database Edit or annotate an existing recipe Plan a meal consisting of several courses Scale a recipe for some number of users Plan a longer period, say a week Generate a grocery list that includes all the items in all the menus for a period

11 11 Characterization by Behavior Just as an Abstract Data Type is characterized more by behavior than by representation The goal in using Responsibility Driven Design will be to first characterize the application by behavior: First capture the behavior of the entire application Refine this into behavioral descriptions of subsystems Refine behavior descriptions into code This emphasis on behavior is a hallmark of Object-Oriented programming

12 12 Working Through Scenarios Because of the ambiguity in the specification, the major tool we will use to uncover the desired behavior is to walk through application scenarios: Pretend we had already a working application Walk through the various uses of the system Establish the look and feel of the system Make sure we have uncovered all the intended uses Develop descriptive documentation Create the high level software design Some people use the term use-cases for this process of developing scenarios

13 13 Software Components A software component is an abstract design entity with which we can associate responsibilities for different tasks May eventually be turned into a class, a function, a module, or something else A component must have a small well defined set of responsibilities A component should interact with other components to the minimal extent possible

14 14 CRC Cards Components are most easily described using CRC cards. A CRC card records the name, responsibilities, and collaborators of n component Component Name Description of the responsibilities assigned Collaborators List of other components to this component

15 15 The first component, The Greeter Let us return to the development of the IIKH The first component your team defines is the Greeter. When the application is started, the Greeter puts an informative and friendly welcome window (the greeting) on the screen Offer the user the choice of several different actions: Casually browse the database of recipes Add a new recipe Edit or annotate a recipe Review a plan for several meals Create a plan of meals Many of the details concerning exactly how this is to be done can be ignored for the moment

16 16 The Recipe Database Component Ignoring the planning of meals for the moment, your team elects to next explore the recipe database component: Must maintain the database of recipes Must allow the user to browse the database Must permit the user to edit or annotate an existing recipe Must permit the user to add a new recipe

17 17 The Who/What Cycle As we walk through scenarios, we go through cycles of identifying a what, followed by a who What action needs to be performed at this moment, Who is the component charged with performing the action Every what must have a who, otherwise it simply will not happen. Sometimes the who might not be obvious at first, i.e., who should be in charge of editing a recipe?

18 18 Postponing Decisions Many decisions, such as the method of browsing, can be ignored for the moment, as they are entirely encapsulated within the recipe database component, and do not effect other components. Scroll bars and windows? A virtual book with thumb-holes and flipping pages? Keywords and phrases? Only need to note that somehow the user can manipulate the database to select a specific recipe

19 19 Responsibilities of a Recipe We can make the recipe itself into an active data structure. It maintains information, but also performs tasks. Maintains the list of ingredients and transformation algorithm Must know how to edit these data values Must know how to interactively display itself on the output device Must know how to print itself We will add other actions later (ability to scale itself, produce integrate ingredients into a grocery list, and so on)

20 20 The Planner Component Returning to the Greeter, we start a different scenario. This leads to the description of the Planner: Permits the user to select a sequence of dates for planning Permits the user to edit an existing plan Associates with Date object

21 21 The Date Component The Date component holds a sequence of meals for an individual date: User can edit specific meals. User can annotate information about dates ( Bob's Birthday, Hanukkah Dinner, and so on) Can print out grocery list for entire set of meals

22 22 The Meal Component The Meal component holds information about a single meal: Allows user to interact with the recipe database to select individual recipes for meals. User sets number of people to be present at meal, recipes are automatically scaled Can produce grocery list for entire meal, by combining grocery lists from individual scaled recipes

23 23 The Six Components Having walked through the various scenarios, your team eventually decides everything can be accomplished using only six software components. Greeter Plan Manager Recipe Database Date Meal Recipe You can at this point assign the different components to different programmers for development

24 24 Interaction Diagrams The picture on the previous slide captures static relationships, but not the dynamic flow of messages in a scenario. That information can be recorded by an interaction diagram: Greeter Database Recipe Planner browse() display() return from display return from browse makepkan()

25 25 Characteristics of Components Let us return to the idea of a software component There are many different aspects to this simple idea, we will consider just a few: Behavior and State Instances and Classes Coupling and Cohesion Interface and Implementation

26 26 Behavior and State All components can be characterized by two aspects: The behavior of a component is the set of actions a component can perform. The complete set of behavior for a component is sometimes called the protocol. The state of a component represents all the information (data values) held within a component. Notice that it is common for behavior to change state. For example, the edit behavior of a recipe may change the preparation instructions, which is part of the state.

27 27 Instances and Classes We can now clarify a point we earlier ignored. There are likely many instances of recipe, but they will all behave in the same way. We say the behavior is common to the class Recipe: Recipe class: Recipe Behavior: Edit Display Recipe instances: Strawberry short cake Vegetables soup Cheese enchilada Since earlier our goal was to identify behavior, we ignored this distinction and concentrated on prototypical objects

28 28 Coupling and Cohesion The separation of tasks into the domains of different components should be guided by the concepts of coupling and cohesion: Cohesion is the degree to which the tasks assigned to a component seem to form a meaningful unit. Want to maximize cohesion Coupling is the degree to which the ability to fulfill a certain responsibility depends upon the actions of another component. Want to minimize coupling

29 29 Interface and Implementation We have characterized software components by what they can do. The user of a software component need only know what it does, not how it does it. Ask not what you can do to a data structure, ask instead what your data structures can do for you

30 30 Two views of a Software System This naturally leads to two views of a software system: Edit a recipe Create a new recipe Display on terminal Text buffer Window pointer Ingredients list The term information hiding is used to describe the purposeful hiding of implementation details

31 31 Parnas' Principles These ideas were captured by computer scientist David Parnas in a pair of principles: The developer of a software component must provide the intended user with all the information needed to make effective use of the services provided by the component, and should provide no other information. The implementer of a software component must be provided with all the information necessary to carry out the given responsibilities assigned to the component, and should be provided with no other information.

32 32 Public and Private View In C++ and Java, Parnas's Principles lead to the ideas of a public and private view: Public view - those features (data or behavior) that other components can see and use Private view - those features (data or behavior) that are used only within the component

33 33 Next Step - Formalize the Interface The next step is to formalize the channels of communication between the components: The general structure of each component is identified. Components with only one behavior may be made into functions Components with many behaviors are probably more easily implemented as classes Names are given to each of the responsibilities - these will eventually be mapped on to procedure names Information is assigned to each component and accounted for Scenarios are replayed in order to ensure all data is available

34 34 A Rose by any other Name Government bureaucrats have long shown us that obscure and idiomatic names can make even the simplest operation sound intimidating. The selection of names is an important task Names should be evocative in the context of the problem Names should be short Names should be pronounceable (read them out load) Names should be consistent within the project

35 35 Documentation Besides CRC cards, it is important that the development of other documentation be performed almost from the beginning. The two most important documents are the user manual and the design documentation of the software system

36 36 User Manual The user manual describes the application as seen by the user. Does not depend upon the implementation, so can be developed before the implementation Can naturally flow from the process of walking through scenarios. Can be carried back to the clients to make sure the users and the implementers have the same ideas

37 37 Quality You should always remember that the primary measure of quality is the degree to which your customers (clients) are satisfied with your product Since often customers do not know exactly what it is they want, it is important to work with the client early in the design phase to make sure the system your are developing is the desired product One very important way to do this is to create the user manual before the software is written

38 38 System Design Documentation Record the decisions made during the process of system design: Record the arguments for and against any major decision, and the factors influencing the final choice Record CRC cards for the major components Maintain a log or diary of the process schedule Important to produce this while the ideas are fresh, not in hindsight when many details will have been forgotten. Note the code only records the outcome of decisions, not factors that lead up to decisions being made

39 39 Preparing for Change Your design team should also keep in mind that change is inevitable. Users requirements change with experience, hardware changes, government regulations change. Try to predict the most likely sources of change, and isolate the effect. Common changes include interfaces, file formats, communication protocols Isolate interfaces to hardware that is likely to change. Reduce dependency of one software component on another Keep accurate record of the reasoning behind every major decision in the design documentation

40 40 Next Step - Select Representations for Subsystems Next the internal representation of the software subsystem corresponding to each component is selected Knowledge of the classic data structures of Computer Science is important here. Often once data structures have been selected, the code is almost self-evident

41 41 Step - Implement and Test Subsystems Classic techniques, such as stepwise refinement, are used to implement each of the subsystems Subsystems are validated in isolation: Informal proofs of correctness for the subsystem are developed Identify necessary conditions for correct functioning. Try to minimize conditions, and test input values whenever possible Software testing is used as a confidence building measure

42 42 Step - Integration and Testing Components are slowly integrated into completed system Stubs can be used to perform testing all during integration Errors discovered during integration to cause reinvestigation of validation techniques performed at the subsystem level.

43 43 Maintainence and Evolution Software does not remain fixed after the first working version is released: Errors or bugs can be discovered. Must be corrected. Requirements may change. Say as a result of government regulations, or standardization among similar products. Hardware may change. Users expectations may change. Greater functionality, more features. Often as a result of competition from similar products. Better documentation may be required. A good design recognizes the inevitability of change, and plans an accommodation for these activities from the very beginning

44 44 Common Design Flaws Direct modification: Components that make direct modification of data values in other components are a direct violation of encapsulation. Such coupling makes for inflexible designs. Too Much Responsibility: Components with too much responsibility are difficult to understand and to use. Responsibility should be broken into smaller meaningful packages and distributed. No Responsibility: Components with no responsibility serve no purpose. Often arise when designers equate physical existence with logical design existence. Money is no object Components with unused responsibility: Usually the result of designing software components without thinking about how they will be used. Misleading Names: Names should be short and unambiguously indicate what the responsibilities of the component involve.