UML Class Diagrams A Review or Brief Introduction

UML Class Diagrams A Review or Brief Introduction Creative Commons License - Curt Hill

Introduction • In this presentation we wish to consider the most common UML diagrams in our trade – These are class diagrams • These are how we rough out the design of programs and systems in an Object Oriented fashion Creative Commons License - Curt Hill

Class • The basic class is a rectangle with one to three pieces • The top portion is the class name • The properties occur in one of the pieces and the methods in another • Properties may be described by attributes or associations • If the class is described already we may leave it at just a name Creative Commons License - Curt Hill

Attribute Notation • Each attribute is described in a way similar to what you might see in a programming language • The general form is: vis name: type mult = def {propstr} – Where • • • Vis is visibility (+ or -) Name and type are obvious Mult is multiplicity Def is default Propstr shows other characteristics • Name is only one that is required Creative Commons License - Curt Hill

Multiplicity • • How many are there? Usually in brackets If absent there is just one This is how an array or other container class would be notated • A range is shown by a two dot ellipsis – [0. . 1] • An * signifies zero or more Creative Commons License - Curt Hill

Visibility • UML is designed to work with any language • Therefore it has four visibility symbols – + public – - private – # protected – ~ package • These may have subtle differences from programming languages – However those differences are not important here Creative Commons License - Curt Hill

Example • Here is the day from the infamous Curt. Date - day: char [1] • Here is another: -title: String = “None” {readonly} • Next we look at a diagram Creative Commons License - Curt Hill

An Order Object Order + received: Date [0. . 1] + is. Prepaid: Boolean [1] + lines: Order. Line[*] {ordered} Creative Commons License - Curt Hill

Associations • An association is a graphical way to do similar things as attributes • A solid line between two classes indicates an association – The contained class has the name, multiplicity and other characteristics on its end Creative Commons License - Curt Hill

Order Again Order received Date 0. . 1 1 * lines {ordered} Order. Line Creative Commons License - Curt Hill is. Prepaid Boolean

Bi. Directional Association • A bidirectional association is also possible • Denoted by double headed arrow • For two items to refer to each other usually implies pointers in C++ – Such as a car object pointing at its owner with a person object pointing at an owned car Creative Commons License - Curt Hill

Implementable Class • How would we implement such a class in C++ or any other language? • Most of the primitive types are obvious • Variables such as lines would likely use a collection or container class • Private variables may have a set/get pair of methods but a readonly variable would have get without set • The next screen is an example Creative Commons License - Curt Hill

One More Time on Order class Order { private: Date received; bool is. Prepaid; list<Order. Line> lines; public: Date get. Received(){ return received; } void set. Received (Date d){ received = d; }. . . }; Creative Commons License - Curt Hill

Methods • Methods are the operations of classes – They implement the behaviors of the class • The notation looks very similar to the attribute notation – Also similar to a prototype notation of some languages – We see what it takes to declare without implementation details Creative Commons License - Curt Hill

Methods Again • The general form is: vis name (parms) : rtype {propstr} • Where – Vis is visibility (+ or -) – Name is obvious – Parms are parameters and use attribute notation – Rtype is return type – Propstr shows other characteristics Creative Commons License - Curt Hill

Parameters Again • One feature that a parameter has that an instance variable or static variable does not is direction • The three directions are: – in – out – inout • Leaving direction off assumes in • Notice that C++ and Java do not distinguish between inout and out but some languages do Creative Commons License - Curt Hill

Setters and Getters • Many classes have set and get methods that give access to a single instance variable • These are usually named something like set. X and get. X – Where X is the variable name • These are usually left out of the UML – They do not tell us much in regards to behavior • Constructors are also often left out Creative Commons License - Curt Hill

Generalization • This is a term that refers to a concept usually implemented with inheritance • An ancestral class is a generalization of descendent classes • If we want the ancestral class we could actually receive the descendent class • Recall the person hierarchy: a person is a generalization of a student or employee Creative Commons License - Curt Hill

Person Example Continued • If a method/function requires a Person – We should be able to pass an employee or student or gradstudent • Similarly, anything that uses a student should be able to accept a gradstudent as well Creative Commons License - Curt Hill

Customer Example • We may have a distinction between customers that are individual people and those that are corporations • We accomplish this by having a Customer class which contains the similarities and derive a Corporate. Customer and Personal. Customer class • We then state that Customer is a generalization of either of these Creative Commons License - Curt Hill

Generalization • Inheritance is usually denoted by the ancestral class (or super class or base class) shown higher • Derived classes (subclasses or subtypes) are below and point to the ancestral class Creative Commons License - Curt Hill

Person Hierarchy Person -name: string get. Age(): Date Employee -wage: float -hours: int Student -hours: int -points: int get. Pay(): float get. GPA (): float Creative Commons License - Curt Hill

Generalization Again • Although UML is supposed to be standard – It is only as standardized as the user • There are variations in form • For example sometimes the generalization arrow is more a triangle to distinguish from other arrows Creative Commons License - Curt Hill

Person Hierarchy Again Person -name: string get. Age(): Date Employee -wage: float -hours: int Student -hours: int -points: int get. Pay(): float get. GPA (): float Creative Commons License - Curt Hill

Notes and Comments • Sometimes we may want some explanation • These are attached in a rectangle with folded corner attached with a dotted line to whatever it describes Creative Commons License - Curt Hill

Commented Class Person -name: string Could be part of U or not get. Age(): Date Creative Commons License - Curt Hill

Dependency • A dependency exists whenever a change to one class mandates a change to another – Change in class structure/interface – Not a change in object value • These occur for several reasons – Class contains another class – Class method uses another class as a parameter – Class calls method of another class Creative Commons License - Curt Hill

Dependencies Again • These are things that need to be monitored • Too many dependencies make a system difficult to maintain – One change has a large ripple effect • Dependencies are shown by a dashed arrow – The dependent item points to that on which it depends Creative Commons License - Curt Hill

Order A Class Example date. Receive: Date is. Prepaid: boolean Price: Money Customer 1 * get. Credit. Rating(): string Dispatch close 1 lineitems * ordered Order. Line Name [1] Address [0. . 1] If credit rating is “poor” then is. Prepaid must be true Prsn. Cust credit. Card Quantity: integer Price: Money Corp. Cust credit. Limit contact. Name month. Bill(int) * 1 Product * sales. Rep 0. . 1 Employee Creative Commons License - Curt Hill

Finally • UML class diagrams are used both for documenting a project and in the design stage • Properties may be represented by attributes or associations • We are mostly interested in groups of classes that are related – Not in just a single class • We may need another dose of this Creative Commons License - Curt Hill