CET203 SOFTWARE DEVELOPMENT Session 1A Revision of Classes

Objectives Introduction to Object Oriented Programming Model a basic class using the UML class diagram notation Implement a class in C# Introduce new naming conventions and C# properties Design and implement a simple multi-class system

Object Orientation Class Object 1 Object 2 Object 3 Object 4 A ‘class’ is a software design which describes the general properties of something which the software is modelling. Individual ‘objects’ are created from the class design for each actual thing

Example A class Car used in a computer system for a second-hand car showroom The class would be a model of the important characteristics of a car for the purposes of the business For each car in the showroom the system would have a separate object – an instance of class Car.

Class Diagram Class name Instance variables Methods The type of object the class is defining (e.g. Car) The information (properties) the objects will record (e.g. model, price) The actions the class can perform (e.g. sell)

An example class - Employee We will create a class to model an employee of a company The company needs to store the following information about each employee – Name – Department – Salary The company determines the salary of an employee based on their profits for the year Therefore the salary can go up or down by a percentage amount each year We will model this in a UML class diagram

UML Class Diagram Employee name : String department : String salary : int IncreaseSalary(percent : int) DecreaseSalary(percent : int) Methods with parameters Instance variables (note the data type comes after the name in the class diagram!)

Sections of a class We can consider a class as containing two distinct sections The interface is the details that the programmer of another class needs to know in order to use the class The implementation is the section that defines how the class actually works A programmer using the class has no need to know about the implementation In fact it is better that they don’t know (or make use of) the details of the implementation Information hiding makes use of this separation

Access modifiers C# provides two keywords to enable information hiding We apply these to the methods and instance variables of the class We use public to identify the interface of the class We use private to identify the implementation of the class These keywords are known as access modifiers In good O-O programming – Instance variables should be private – Methods which are called from outside the class are public – Methods which are only used within the class (helper methods) are private

Access Modifiers on a class diagram Employee - name : String - department : String - salary : int + IncreaseSalary(percent : int) + DecreaseSalary(percent : int) + denotes public - denotes private (note the data type comes after the name in the class diagram!)

Accessor/Mutator methods In CET101 we gained access to an instance variable from outside the class using two special methods An accessor method or “getter” returned the value of an instance variable public String getName() { return name; } A mutator method or “setter” modified the value of an instance variable public String setName(String pName) { name = pName; }

An alternative approach in C# Accessor (getter) and mutator (setter) methods can be replaced with an alternative called Properties Properties are defined with a special code syntax for getting/setting the instance variable Properties often have the same name as the instance variable but with an upper-case first letter Our convention will be to remove the m_ and the data type character from instance variables (as used in CET101) and use case to distinguish between the instance variable and the property For example the property associated with instance variable name will be called Name

Defining a property private String name; public String Name { get { return name; } set { name = value; } Instance variable Property Obtain value from instance variable Assign value to instance variable

Auto-implemented properties Any number of statements can be included in the get/set accessors but often they simply return or assign a value C# 3.0 introduced a more concise format for properties private String name; public String Name {get; set}; The properties implementation is created automatically with the assumptions – The set accessor simply assigns a value to the appropriate field – The get accessor simply returns the value of the appropriate field

Creating objects and using properties Employee emp1 = new Employee(); emp1.Name = "Liz Gandy"; Console.WriteLine("Employee name:” + emp1.Name); Create object Assign value to instance variable via property Obtain value from instance variable via property

Calling Methods To call a method we link it to the reference name using the “dot” notation: emp1.IncreaseSalary(); If the method requires parameters we place these in the brackets (as we would for functions): emp1.IncreaseSalary(10); If the method requires multiple parameters then we separate them with commas emp1.IncreaseSalary(10, "monthly");

Constructors Constructors are special methods which are called when an object of the class is first created (using new) They are always defined as public and have the same name as the class They should not return a value They may require parameters which are used to initialise instance variables or they may initialise them to default values Classes can contain multiple constructors with different parameter sets The programmer can then choose whichever is most appropriate for their needs

Employee Class Constructors //constructor with no parameters public Employee() { name = "No name"; department = "Not set"; salary = 0; } //constructor with parameters public Employee(String name, String dept, int salary) { this.name = name; this.department = dept; this.salary = salary; } Note the use of this to distinguish instance variable from parameter

Creating objects using constructors Using the constructor with no parameters: Employee emp1 = new Employee(); Using the constructor with parameters Employee emp2 = new Employee("Jane Smith", "Accounts", 21500);

Demonstration EmployeeClass.sln

Designing multiple class systems We have seen that object oriented systems model the real world Systems in the real world normally contain many entities inter-linked with each other In an OO model these entities will be represented by classes A real OO system will therefore comprise multiple classes These classes will have “relationships” between them

The Bank/Employee System We will now incorporate our Employee class into a banking/employee system A bank will contain branches, each located in a particular town or city Each branch will employ staff These employees will be assigned to a particular department and receive a particular salary Depending on the banks profits and employee performance their salaries may increase or decrease each year

The Branch class Branch - location : String - employeeCount : int + AddEmployee(employee : Employee) + GetEmployee(index : int) : Employee

Relationships We need to identify the relationships in our system The Branch “uses” Employee(s) We can represent this on a simplified class diagram The arrow indicates the relationship between the classes BranchEmployee

References Relationships are represented by a special type of variable We call this a reference to another class If ClassA uses ClassB then we provide a reference in ClassA References are declared just like other variables The data type of the reference variable will be the name of the class it refers to The Branch class requires a reference to the Employee class In this case because multiple employees may work for a branch we need the reference to be an array Employee [] employeeList; We will use employeeList to store Employee objects and access the methods of the Employee class

The full Class Diagram Branch - location : String - employeeCount : int + AddEmployee(employee : Employee) + GetEmployee(index : int) : Employee Reference is placed here Employee - name : String - department : String - salary : int + Employee(name : String, dept : String, salary : int) + IncreaseSalary(percent : int) + DecreaseSalary(percent : int) employeeList[ ] Note: parameterised constructors are normally shown on the class diagram

Coding the Branch class The reference is declared like any other instance variable private Employee[] employeeList; private String location; private int employeeCount; The actual object is created in the constructor public Branch(String location) { employeeList = new Employee[MAXEMPLOYEES]; this.location = location; employeeCount = 0; }

Adding new employees Objects can be passed into methods as parameters and used like any other variable/parameter public bool AddEmployee(Employee employee) { if (employeeCount < MAXEMPLOYEES) { employeeList[employeeCount] = employee; employeeCount++; return true; } else { return false; } Because the employee list is an array we need make sure we don’t overflow it’s size!

Returning a chosen employee Methods can also return values which are objects public Employee GetEmployee(int index) { if (index < employeeCount) { return employeeList[index]; } else { return null; }

Demonstration Bank.sln