Introduction to UML-Basics of Software Engineering-Lecture Slides, Slides of Software Engineering

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Introduction to UML  During this lecture, we shall see how various features of the Unified Modeling Language (UML) can help to reduce ambiguities and increase understanding of a proposed system  We will be studying an introductory case study based on a library system and giving an introduction to:  Use case diagrams  Class diagrams  Sequence diagrams  State diagrams 1 COMP201 - Software Engineering docsity.com The Problem  The most difficult part of any design project is understanding the task you are attempting  Example: you have been contacted to develop a computer system for a university library.  The library currently uses a 1960s program, written in an obsolete language, for some simple bookkeeping tasks, and a card index for user browsing.  You are asked to build an interactive system which handles both of these aspects online. 2 COMP201 - Software Engineering docsity.com Use Case Model  If a system is to be seen as having high quality, it must meet the needs of its users.  So we take a user-oriented approach to systems analysis.  We identify the users of the system and the tasks they must undertake with the system.  We also seek information about which tasks are most important, so that we can plan the development accordingly. 5 COMP201 - Software Engineering docsity.com What do we Mean by “Users” and “Tasks”?  UML uses as technical terms “actors” and “use cases”  An actor is a user of a system in a particular role (an actor can also be an external system)  For example. Our system will have an actor BookBorrower representing the person who interacts with the system to borrow a book  A use case is a task which an actor needs to perform with the help of the system,  such as Borrow copy of Book. 6 COMP201 - Software Engineering docsity.com 7 Use Case Diagram for the Library COMP201 - Software Engineering docsity.com Use Case Diagram for the First Iteration  Let us suppose that after discussing priorities with the customers we decide that the first iteration of the system should provide:  Borrow copy of book, Return copy of book, Borrow journal, Return Journal 10 COMP201 - Software Engineering docsity.com Use Case Advantages  It may be easier to identify the amount of time required to implement all the required features of the system.  Such details can often be optimistically overlooked.  We can identify which requirements are important to key strategic (or most influential) personnel in the company.  By providing this functionality early, we can show the potential value of the software and avoid the project being cancelled. COMP201 - Software Engineering 11 docsity.com Use Case Advantages  We may decide to implement more risky use cases first since we would hopefully still have contingency to tackle problems that arise  In other words, early in the development we have more flexibility in terms of time, money, design choices etc.  Use cases can be used to derive validation checks on the developed system, in order that it provides all required functionality. COMP201 - Software Engineering 12 docsity.com Removing Superfluous Candidates  In this particular case, we discard:  Library, because it is outside the scope of our system  Short term loan, because a loan is really an event, which so far as we know is not a useful object in this system  Member of the library, which is redundant  Week, because it is a measure, not a thing  Item, because it is vague (we need to clarify it)  Time, because it is outside the scope of the system  System, because it is part of the meta-language of requirements description, not a part of the domain  Rule, for the same reason 15 COMP201 - Software Engineering docsity.com This leaves:  Book  Journal  Copy (of book)  Library member  Member of staff 16 COMP201 - Software Engineering docsity.com Relations between Classes  Next we identify and name important real-world relationships or associations between our classes  We do this for two reasons:  To clarify our understanding of the domain, by describing our objects in terms of how they work together;  To sanity-check the coupling in our system, i.e. make sure that we are following good principles in modularising our design  We shall now see the relations for the library system example.. 17 COMP201 - Software Engineering docsity.com Revised Library Class Model 20 COMP201 - Software Engineering docsity.com The System in Action  A class diagram gives a static view of the system, but we know nothing about the dynamic behaviour  In UML we can use interaction diagrams to show how messages pass between objects of the system to carry out some task  This will also show how the various classes realize the different use cases we identified in the use case diagram 21 COMP201 - Software Engineering docsity.com An Example Sequence Diagram  Consider what happens in the library scenario when a user wishes to borrow a copy of a book.  The library member must first check that they are able to borrow a book (i.e., they have borrowing privileges, there exists at least one remaining copy of the book etc.)..  Then the copy object must be informed that the library member wishes to borrow the copy of the book..  Finally the copy object must then tell the individual book object that it is out on loan and thus no longer available.  We now see how this is recorded in a sequence diagram 22 COMP201 - Software Engineering docsity.com State Diagrams  Objects in the system have a state which is all the data which it currently encapsulates  For example, a Book object in the library system can be “borrowable” or “not borrowable”.  Running methods on the object can cause a change in state, i.e., by borrowing the book we change its internal state.  Changes in object states can be modelled by a state diagram.. COMP201 - Software Engineering 25 docsity.com Changes of the System: State Diagrams  State diagram for class Book  We will consider state diagrams again in more detail in a later lecture 26 COMP201 - Software Engineering docsity.com Lecture Key Points  We have seen an introduction to the Unified Modelling Language (UML)  We studied an introductory case study based on a library system with an introduction to:  Use case diagrams  Class diagrams  Sequence diagrams  State diagrams COMP201 - Software Engineering 27 docsity.com