Analysis to Design
Input
- The output of object oriented analysis is provided as input to object oriented design.
- Analysis and design may occur in parallel with incremental and iterative process.
- Conceptual model, use case, system sequence diagram, UI documentation and relational data model are the input artifacts.
Design
- With the help of the input artifacts, we can conclude the following:
1. Define object and class diagram from conceptual model.
2. Attributes are identified.
3. A description of solution to a common problem (design patterns) are used.
4. Application framework is defined.
5. The persistent objects or data are identified.
6. The remote objects are identified and defined.
Output
- Sequence diagram and design class diagram are the typical output artifacts of object oriented design.
- The comparison of analysis model and design model is shown in given table:
| Analysis Model | Design Model |
|---|---|
|
|
Describing and Elaborating Use Cases
Class Responsibility Collaboration (CRC) Cards
- It is a text oriented modelling techniques or tools used in design of object oriented software.
- It is a paper index cards in which the responsibilities and collaborators of classes are written.
- Each card represents a single class.
- The general format of CRC card is shown below:
| Class Name | |
| Responsibilities | Collaborators |
Realization of Use Cases
- It represents the implementation of use case in terms of collaborating objects.
- It may include textual document, class diagrams of participating classes and interaction diagrams.
UML Interaction Diagram
- Interaction diagram shows how objects interact via messages.
- It is used for dynamic object modelling.
- It is of two types. They are as follows:
1. Sequence diagram
2. Communication or collaboration diagram
Sequence Diagram
- A sequence diagram shows the interaction among objects as a two dimensional chart, in a fence format, in which each new object is added to the right.
- The objects participating are shown at the top of the chart as boxes attached to a vertical dashed line.
- The name of object with a semicolon separating it from the name of the class is written inside the box.
- The name of object and the class is underlined.
- The vertical dashed line indicates the lifeline of objects.
- The rectangle drawn on the lifeline is called activation symbol, which indicates that the object is active as long as the rectangle exists.
- Each message is indicated as an arrow between the lifeline of two objects.
- The messages are shown in chronological order.
- Each message is labeled with the message name.
Advantages:
- It shows the time sequence of messages.
- It has large set of detailed notation options.
Disadvantages:
- It is forced to extend to right when new objects are added, which consumes horizontal space, making it difficult to view by the user.
Basic Notations
Collaboration Diagram
- Collaboration diagram shows both structural and behavioral aspects.
- Structural aspect includes objects and link between them.
- It illustrates the object interactions in a graph format in which objects can be placed anywhere.
- The link between objects is shown as a solid line.
- The messages are labelled with a arrow and prefixed with sequence number.
Advantages:
- It is flexible to add new objects.
Disadvantages:
- It is difficult to know the sequence of messages.
- It includes fewer notation options.
Basic Notations:
1. Class
| Sale |
2. Instance
| :Sale |
3. Named instance
| S1 : Sale |
4. Messages
seq no return := message (parameter : parameterType) : returnType ---------------->
5. Links
---------------------------------
6. Conditional messages
1[color = red] : calculate() --------------->
7. Iteration
1 * [r = 1 .. N] : num := nextInt()
Objects and Patterns
Patterns
- Patterns is a way of doing something.
- Design pattern is a category of patterns that deals with object oriented software.
- Design pattern is a general repeatable solution to a commonly occurring problem in software design.
- It is a description for how to solve a problem that can be used in many different situations.
Components
1. Pattern Name
2. Pattern Description
3. Solution
4. Consequences
Advantages
- It reduces a complex form to a simple one.
- It provides clear concept of the problem and its solutions.
- It provides a mean to model a good solution.
- It facilitates communication.
Usage
- It can speed up the development process, so used in Rapid Application Development (RAD)
- It provides a general solution, so can be used in any platform.
GRASP Design Pattern
- GRASP stands for General Responsibility Assignment Software Patterns.
- These principles are important to design object oriented software.
- It guides choices in where to assign responsibilities.
1. Information Expert:
- Problem : Which class should be responsible for doing certain things?
- Solution : Assign responsibility to the class that has the information necessary to fulfill the required responsibility.
- It describes that the objects do things related to the information they have.
2. Creator:
- Problem : Who should be responsible for creating a new instance of some class?
- Solution : Assign class B the responsibility to create an instance of class A if one or more of the following is true:
a) B aggregates A objects.
b) B contains A objects
c) B records instances of A objects
d) B closely uses A objects
- If more than one option applies, prefer a class B which aggregates class A.
3. High Cohesion:
- Problem : How to keep objects focused, understandable and manageable?
- Assign responsibility so that cohesion remains high.
- Cohesion is a measure of how strongly the responsibilities of an element are related.
- An element with highly related responsibilities and which does not do a tremendous amount of work has high cohesion.
- The classes with low cohesion suffers from:
a) hard to comprehand
b) hard to reuse
c) hard to maintain
d) constantly affected by change
4. Low Coupling:
- Problem : How to support low dependency, low change impact and increased reuse?
- Assign responsibility so that coupling remains low.
- Coupling is a measure of how strongly one element is connected to other elements.
- The class with high coupling suffers from:
a) forced local changes
b) harder to understand in isolation
c) harder to reuse
5. Controller:
- Problem : What first object beyond UI controls a system operation?
- Assign responsibility to a class representing one of the following choices:
a) Represents overall system
b) Represents a use case scenario within which system events occur
Frameworks
- Framework is a group of concrete classes which can be directly implemented on an existing platform.
- They are written in programming languages.
- They are concerned with specific application domain.
Patterns vs Frameworks
- Pattern is a concept while framework is code to be used.
- Pattern supports reuse of software architecture and design while framework supports reuse of detailed design and code.
Model View Controller (MVC) Framework
- It is a software architectural pattern to implement UI.
- It divides the given software application into three inter connected parts, to separate internal representation of information from the ways they are presented to the users.
- The central component is model that captures the behavior of the application independent of UI. It manages data, logic and rules of the application.
- A view can be any output representation of information visible to the users.
- The controller accepts input and converts it to commands for the model or view. It links model and view together.
Determining Visibility
Visibility
- It is the ability of an object to have a reference to another object.
- It indicates the scope of the objects.
- For a sender object to send message to a receiver object, the sender must be visible to the receiver.
Attribute Visibility
- Attribute visibility from A to B exists when B is an attribute of A.
- It is relatively permanent i.e. it persists as long as A and B both exists.
Parameter Visibility
- Parameter visibility from A to B exists when B is passed as a parameter to a method of A.
- It is relatively temporary i.e. it exists within the scope of the method.
- It is generally transformed into attribute visibility within a method.
Local Visibility
- Local visibility from A to B exists when B is declared as a local object within a method of A.
- It is temporary as it persists only within the scope of the method.
- It can be achieved by:
1. Create a new local instance and assign it to local variable.
2. Assign returning object from a method invocation to a local variable.
Global Visibility
- Global visibility from A to B exists when B is global to A.
- It is permanent.
- It can be achieved by:
1. Assign an instance to a global variable.
Visibility in UML
1. Public
- Any outside classifier with the visibility to the given classifier.
- It is denoted by '+'
2. Protected
- Any descendant of classifier can use this feature.
- It is denoted by '#'
3. Private
- Only the classifier itself can use the feature.
- It is denoted by '-'
Visibility Example
The example that shows details of the visibility is shown in figure below:
Class Diagram
- Class diagram is used for static modeling that illustrates classes, interfaces and their relationships.
- It shows the structural view of the system.
Types of Relationships
1. Association:
- It enables objects to communicate with each other.
- It describes a connection between classes.
- A link is the physical or conceptual connection between object instances.
- The association relationship of a class with itself is known as recursive association.
- It is represented by drawing a straight line between concerned classes.
- Arrow head can be used to indicate reading direction.
- The multiplicity is noted on each side.
2. Aggregation:
- It is the association in which the involved classes represent a whole-part relationship.
- It takes the responsibility of leadership.
- When an instance of one object contains instances of some other objects, then aggregation exists between composite object and component object.
- It is represented by a diamond symbol at the end of a relationship.
- It can never be recursive and symmetric.
3. Composition:
- It is the strict form of aggregation, in which the parts are existence-dependent on the whole.
- It is represented as a filled diamond drawn at composite end.
- The part instance can only be the part of one composite at a time.
- Creation and deletion of parts is managed by composite.
4. Inheritance (Generalization and Specialization):
- It describes 'is a kind of' relationships between classes.
- Object of derived class inherits the properties of base class.
- It is defined statically.
5. Dependency:
- It indicates that one element has knowledge of another element.
- It states that a change in specification of one thing may affect another thing using it, but not necessarily the reverse.
- It depicts non-attribute visibility between classes.
6. Realization:
- It indicates the implementation of functionality defined in one class by another class.
Attributes
- The format of attribute is given as:
visibility name : type multiplicity = default {property - string}
Association Class
- It allows you treat an association itself as a class.
Constraints
- It is visualized in text between braces.
Qualified Association
- It is the association with a qualifier.
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