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Unified Modeling Language (UML)

UML Definition

• Used to model the Object-Oriented Analysis of a software system.
• A way of visualizing and documenting a software system by using a collection of diagrams.
• It helps engineers, businesspeople and system architects understand the behaviour and structure of system being designed.

Benefits of using UML

• Helps develop a quick understanding of a software system.
• UML modeling helps in breaking a complex system into discrete pieces that can be easily understood.
• UML’s graphical notatioins can be used to communicate design decisions.
• As UML is independent of any specific platform or language or technology, its easier to abstract out concepts.
• Becomes easier to hand over the system to new team.

Types of UML Diagrams

• Current UML standard has 14 different kinds of diagrams.
• Structural UML Diagrams: These diagrams analyze and depict the structure of a system or process.
  • Class Diagram
  • Object Diagram
  • Package Diagram
  • Component Diagram
  • Composite Structure Diagram
  • Deployment Diagram
  • Profile Diagram
• Behavioural UML Diagrams: These diagrams describe the bevaviour of the system, its actors and its building components.
  • Use Case Diagram
  • Activity Diagram
  • Sequence Diagram
  • State Diagram
  • Communication Diagram
  • Interaction Overview Diagram
  • Timing Diagram

Important UML Diagrams to be discussed

• Use Case Diagram: Used to describe a set of user scenarios, it illustrates the functionality provided by the system.
• Class Diagram: Used to describe structure and behaviour in the use cases, provides a conceptual model of the system in terms of entities and their relationships.
• Activity Diagram: Used to model the functional flow-of-control b/w two or more class objects.
• Sequence Diagram: Used to describe interactions among classes in terms of an exchange of messages over time.

Use Case Diagrams

Use Case Diagrams Definition

• Use case diagrams describe a set of actions (called use cases) that a system should or can perform in collaboration with one or more external users of the system (called actors).
• Each use case should provide some observable and valuable results to the actors

Functionality of Use Case Diagrams

• It describes the high-level functional behaviour of the system.
• It answer what system does from the user point of view.
• It tells “What will the system do?” and “What it will not do?”.

###### Use Case Definition

• A use case illustrates a unit of functionality provided by the system.
• Primary purpose of the use case diagrams is to help development teams visualize the functional requirements of a system.
• It also includes relationship of actors to the essential processes, as well as relationship among different use cases.

Drawing a Use Case Diagram:

• To illustrate a use case on the diagram, draw an oval and put the name of use case in the oval.
• To show an actor (user of the system) on the diagram, draw a stick figure to the left or right of diagram.

Different Components of Use Case Diagram

• System Boundary:
  • Defines the scope and limits of the system.
  • Shown as rectangle that spans all use cases of the system.
• Actors:
  • An actor is an entity that performs specific actions.
  • The roles are actual business roles of the users in a given system.
  • An actor interacts with a use case of the system.
  • Example:- Banking System - Customer is one actor, employee is another actor etc.
• Use Case:
  • Every business functionality is a potential use case.
  • The use case should list the discrete business functionality specified in the problem statement.
• Include Relationship:
  • Represents an invocation of one use case by another use case.
  • From coding perspective its like one function being called by another function.
• Extend Relationship:
  • Signifies that the extended use case will work exactly like base use case with some added steps.

Class Diagram

Class Diagram Definition

• It is a structural UML diagarma and is the backbone of object oriented modeling.
• It shows how different entities (people, things and data) relate to each other.
• It describes the attributes and operations of a class and also the constraints imposed on the system.
• Widely used in the modeling of object-oriented systems because they are the only UML diagrams that can be directly mapped to code.

Purpose of Class Diagram

• Analysis and design of the static view of an application
• To describe the responsibilites of the system
• To provide a base for component and deployment diagrams
• Forward and Reverse Engineering

Depiction of Class in Class Diagram

• A class is depicted with three horizontal sections in a class diagram.
  1. Upper Section: Name of Class
  2. Middle Section: Properties of the Class
  3. Lower Section: Operations (Methods) on Class

Relationship b/w Classes

1. Association

• If two classes need to communicate with each other there must be a link b/w them and this link is represented by association.
• Associations can be represented in a class diagram by a line b/w these classes with an arrow indicating the navigation direction.
• By default, associations are always bi-directional, that means both classes are aware of each other and their relationship.
  • Example: Association b/w Pilot and FlightInstance
• In uni-directional association, two classed are related but only once class knows that the relationship exists.
  • Example: Only flight class knows about Aircraft

2. Multiplicity

• Multiplicity indicates how many instances of a class participate in the relationship.
• It is a constraint that specifies the range of permiited cardinalities b/w two classes.
• A ranged multiplicity can be expressed as 0....* which means “zero to many” or as 2....4 which means “two to four”.
• We can indicate the multiplicity of an association by adding multiplicity adornments to the line denoting the association.
  • Example: In the diagram below one FlightInstance will have two Pilots, while a Pilot can have many FlightInstances.

3. Aggregation

• Aggregation is a special type of association used to model a “whole to its parts” relationship.
• Aggregation implies a relationship where the Child class can exist independently of the Parent class.
  • Example: Aircraft can exist without Airline.

4. Composition

• Composition relationship is just another form of aggregation relationship.
• In this Child class is dependent on the Parent class .
• Child class can not exist indepenedtly of the Parent class.
  • Example: WeeklySchedule is composed in Flight and when Flight lifecycle ends, WeekSchedule automatically gets destroyed.

5. Generalization

• It is the mechanism for combining similar objects into a single, more general class.
• It identifies commonalities among a set of entities .
  • Example: Crew, Pilot and Admin all are Person.

6. Dependency

• It is a relationship in which one class the Client class uses or depends on another class Supplier class.
  • Example: FlightReservation depends on Payment.

7. Abstract Class

• An abstract class is identified by specifying its name in italics.
  • Example: Person and Account classes are abstract.

Sequence Diagram

Sequence Diagram Definition

• Describe interactions among classes in terms of an exchange of messges over time.
• It is used to explore the logic of complex operations, functions or procedures.
• The sequence of interactions b/w the objects is represented in a step-by-step manner.
• Sequence diagram shows a detailed flow for a specific use case or even just part of a particular use case.
• They are almost self explanatory and show the calls b/w the different objects in their sequence.
• They can also explain at a detailed level different calls to various objects.

Depiction of Sequence Diagram

• A sequence diagram has 2 dimensions vertical and horizontal.
• Vertical dimension shows the sequence of messages in the chronological order thay they occur.
• Horizontal dimension shows the object instances to which the messages are sent.

Drawing Sequence Diagram

• At the top of the diagram - identify the class instances (objects) by putting each class instance inside a box (see above).
• If a class instance sends a message to another class instance, draw a line with an open arrowhead pointing to the receiveing class instance and place the name of the message above the line.
• Optionally for important messages, we can draw a dotted line with an arrowhead pointing back to the originating class instance and label the returned value above the dotted line.

Activity Diagrams

Activity Diagram Definition

• Activity diagrams are used to illustrate the flow of control in system.
• An activity diagram shows the flow of control for a system functionality.
• It emphasizes the condition of flow and the sequence in which it happens.
• We can also use an activity diagram to refer to the steps involved in the execution of a use case.

Purose of Activity Diagrams

• It illustrate the dynamic nature of a system by modeling the flow of control from activity to activity.
• An activity represents an operation on some class in the system that results in a change in the state of the system.
• Typically activity diagrams are used to model workflow or business processes and internal operations.

Difference b/w Activity Diagram and Sequence Diagram

• Activity Diagram:
  • Captures the process workflow and is used for funtional modeling.
  • Functional model represents the flow of values from external inputs through operations & internal data stores to external outputs.
• Sequence Diagram:
  • Tracks the interaction b/w the objects and is used for dynamic modeling.
  • Dynamic Modeling is represented by tracking states, transition b/w states and the events that trigger these transitions.

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