CSSE 501 Object-Oriented Development

Today…  Chapter 3: Object-Oriented Design

Programming in the Small and Programming in the Large

Chapter 3: Object-Oriented Design  Software development Ultimate goal: deliver software products that are oriented towards  Customer satisfaction  Customer delight  Customer ecstasy

Popular software development methodologies  System Development Life Cycle (SDLC) Model  Prototyping Model  Rapid Application Development Model  Component Assembly Model

System Development Life Cycle (SDLC) Model  Waterfall model 1.System/Information Engineering and Modeling 2. Software Requirement Analysis 3. System Analysis and Design 4. Code Generation/ Implementation 5. Testing 6. Maintenance

Prototyping Model  A cyclic version of the linear model  Once the requirement analysis is done and the design for a prototype is made, the development process gets started to create a prototype  Once the prototype is created, it is given to the customer for evaluation  Many iterations between customers’ feedback and developers’ refinement until the final software package is delivered  In this methodology, the software is evolved as a result of periodic shuttling of information between the customer and developer  Popular development model in the contemporary IT industry - as it is very difficult to comprehend all the requirements of a customer in one shot  New versions of a software product evolve as a result of prototyping

Prototyping Model( many versions) or

Rapid Application Development (RAD) Model  A "high speed" adaptation of the linear sequential model  Achieved by using a component-based construction approach  The RAD approach encompasses the following phases: Business modeling  The information flow among business functions is modeled in a way that answers the following questions: What information drives the business process? what information is generated? Who generates it? where does the information go? who processes it? Data modeling  The information flow defined as part of the business modeling phase is refined into a set of data objects that are needed to support the business  The characteristic (called attributes) of each object is identified and the relationships between these objects are defined

Rapid Application Development (RAD) Model (Cont.) Process modeling  The data objects defined in the data-modeling phase are transformed to achieve the information flow necessary to implement a business function  Processing the descriptions are created for adding, modifying, deleting, or retrieving a data object Application generation  The RAD model assumes the use of the RAD tools like VB, VC++, Delphi etc... rather than creating software using conventional third generation programming languages  The RAD model works to reuse existing program components (when possible) or create reusable components (when necessary)  In all cases, automated tools are used to facilitate construction of the software Testing and turnover  Since the RAD process emphasizes reuse, many of the program components have already been tested. This minimizes the testing and development time

Component Assembly Model  Object technologies provide the technical framework for a component-based process model for software engineering  The object oriented paradigm emphasizes the creation of classes that encapsulate both data and the algorithm that are used to manipulate the data  If properly designed and implemented, object oriented classes are reusable across different applications and computer based system architectures  Component Assembly Model leads to software reusability. The integration/assembly of the already existing software components accelerate the development process  Nowadays many component libraries are available on the Internet. If the right components are chosen, the integration aspect is made much simpler

What/Why is Software Design  Software design sits at the crossroads of all the computer disciplines: hardware and software engineering, programming, human factors research, ergonomics. It is the study of the intersection of human, machine, and the various interfaces—physical, sensory, psychological— that connect them --- by Association for Software Design (ASD) members  Design is conscious  Design keeps human concerns in the center  Design is a dialog with materials  Design is creative  Design is communication  Design is a social activity  Design has social consequences

Basis for Design  Consider for the moment what aspects of a problem (i.e., to develop a system) 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

Design Notations/Modeling Languages  Many artifacts generated during the process of software development Requirement specifications Design documents Source code User manual  Design notations capture design documents Informal (natural languages) Unified Modeling Languages (UML, semi-formal) Formal (formal methods, architecture description languages)

Object-Oriented Design Techniques/Methods  Object-Oriented Development (OOD)/Booch  Hierarchical Object-Oriented Design (HOOD)  The Object Modeling Technique (OMT)  Responsibility-Driven Design (RDD)/Class- Responsibility-Collaboration (CRC)  Object-Oriented Analysis (OOA)  …

Object-Oriented Development (OOD)/Booch  Define the problem  Develop an informal strategy for the software realization of the real world problem domain  Formalize the strategy Identify the classes and objects at a given level of abstraction Identify the semantics of these classes and objects Identify the relationships among these classes and objects Implement these classes and objects

Object-Oriented Development (OOD)/Booch (Cont.)  Major advantages Rich notation available:  class diagrams (class structure - static view)  object diagrams (object structure - static view)  state transition diagrams (class structure - dynamic view)  timing diagrams (object structure - dynamic view)  module diagrams (module architecture)  process diagrams (process architecture)

Object-Oriented Development (OOD)/Booch (Cont.) A Booch Class Diagram for a Company

The Object Modeling Technique (OMT)  A method which leads to three models of the system corresponding to three different views of the system The object model  Describes the static structure of the objects in a system and their relationships  Main concepts are: class, attribute, operation, inheritance, association (i.e. relationship), aggregation The dynamic model  Describes the aspects of the system that change over time  Used to specify and implement the control aspects of a system  Main concepts are: state, sub/super state, event, action, activity The functional model  Describes the data value transformations within a system  Main concepts are: process, data store, data flow, control flow, actor (source/sink)

The Object Modeling Technique (OMT)(Cont.)  The method is divided into four phases, which are stages of the development process Analysis  The building of a model of the real world situation, based on a statement of the problem or user requirements System design  The partitioning of the target system into subsystems, based on a combination of knowledge of the problem domain and the proposed architecture of the target system (solution domain) Object design  Construction of a design, based on the analysis model enriched with implementation detail, including the computer domain infrastructure classes Implementation  Translation of the design into a particular language or hardware instantiation, with particular emphasis on traceability and retaining flexibility and extensibility

The Object Modeling Technique (OMT)(Cont.) An Object Model of a Company

Responsibility Driven Design (RDD)  Developed as Rebecca Wirfs-Brock  One of object-oriented design techniques, driven by an emphasis on behavior at all levels of development  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

Responsibility Driven Design (RDD)  Describing the actions and activities for which our software is responsible  Describing the responsibilities in terms that both users and developers can understand  Designing software objects that implement those responsibilities

Responsibility Driven Design (RDD)  RDD is not a sequential process  Steps in RDD Software components Formalize interfaces Designing and representations Implementing components Integration of components Maintenance and evolution

Case Study: the IIKH  Briefly, the Intelligent Interactive Kitchen Helper (IIKH) will replace the box of index cards of recipes in the average kitchen

Your Job  Imagine you are the chief software architect  Your job is to develop the software that will implement the IIKH

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

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

Working Through Scenarios  Because of the ambiguity in the specification, the major tool here used to uncover the desired behavior is to walk through application scenarios Pretend there is already a working application Walk through the various uses of the system  Establish the “look and feel'' of the system  Make sure uncover all the intended uses  Develop descriptive documentation  Create the high level software design  The term “use-cases'' used for this process of developing scenarios by others

Example: Browsing Scenario  Alice Smith starts the IIKH  IIKH displays welcome message  Alice presses the return button to begin  Alice is given a choices of a number of options  Alice selects to browse recipe  Alice enters keywords to search, e.g., salmon, dill weed  IIKH return two results  Alice selects the first  IIKH displays a window, showing everything regarding the recipe  Alice does not want the recipe, returns to the search result page  Alice selects the second recipe  Alice wants the recipe  Alice selects “quit” from the IIKH program menu  The IIKH program quits

Software Components  A software component is simply 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

CRC (Component, Responsibility, Collaborator) Cards  Components are most easily described using CRC cards  A CRC card records the name, responsibilities, and collaborators of an component Inexpensive, Erasable, Physical

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

The Greeter

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

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?

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

Responsibilities of a Recipe  We make the recipe itself into an active data structure. It maintains information, but also performs tasks Maintains the list of ingredients and transformation algorithm (i.e., ingredients to final product) 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)

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

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'', “Christmas Dinner'', and so on) Can print out grocery list for entire set of meals

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

The Six Components  Having walked through the various scenarios, you team eventually decides everything can be accomplished using only six software components  You can at this point assign the different components to different programmers for development

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

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

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

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

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

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''.

Two views of a Software System  This naturally leads to two views of a software system  The term information hiding is used to describe the purposeful hiding of implementation details

Parnas' Principles  These ideas were captured by computer scientist David Parnas in a pair of rules, which are known as Parnas' 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

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

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

Names with Activities  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 Avoid digits within a name

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

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

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 even before the software is written

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

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

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

Next 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

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

Maintenance 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

Common Design Flaws  The following categories present some of the more 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