AK IT: Softwarekonstruktion 1

Session 04:C# OOP 2

OOP in C#:Object-Oriented Design.

Realisation of Object Associations:1-1, 1-n, n-m.

UML Class Diagram.More about Object Interaction.

Collections of Objects

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The Anatomy of a Class

Classes are usually written by this pattern:

class ClassName {

declaration of attributesconstructorspropertiesmethods

}

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The Class BankAccount- attributes and constructor

namespace Banking{

public class BankAccount{

private double balance;private int accNo;private int interestRate;

public BankAccount(int no, int ir){

balance = 0;accNo = no;intrestRate = ir;

}

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Methods

public bool Withdraw(double amount)

public void Deposite(double amout)

public void GiveInterest()

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Properties

public int InterestRate{

get{return interestRate;}set{if( value>=0) interestRate = value;}

}

Object Interaction

• Objects may be connected in different ways:– Association (“know-of-relation”).

One object uses another.

– Aggregation (“part-of-relation”).One object is a part of another.

• The distinction is not always clear .

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Means aggregation

Cardinality or Multiplicity• Tells how many other objects an

object may be associated with:– One customer may have one

account, an account must belong to a customer.

– One customer may have many accounts, an account must belong to one customer.

– A customer may one or more accounts, an account may belong to one or more customers.

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aggregation as well.

Object Interaction: 1 - 1

• The Banking example shows object interaction:– The classes are connected – in UML:

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In the Code

public class Customer{//…private BankAccount account;//…

account= new BankAccount(no, ir, bal);

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Customer is responsible for creating BankAccount objects.

The association is implemented by an

object reference (attribute).

In the Code

public class Program{ //… Customer c = new Customer(1, "Peter Thomsen"); //…

Console.WriteLine("Customer: "+ c.Name +" has DKK “ + c.Account.Balance + " in the bank");

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Methods in the other class is called using the reference.

Forest Exercise

• Solution?

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Implementing 1 - n• One customer may have many

accounts, an account must belong to one customer.

• Possible solution: A collection of BankAccounts in Customer (accounts)

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In the Code

public class Customer{ //…

private List<BankAccount> accounts; //… public Customer(int cNo, string n){

//… accounts = new List<BankAccount>();

}public void AddAccount(BankAccount acc){

accounts.Add(acc);}

//…

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View Source

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Pair Programming(an eXtreme Programming practice)

• A well known and widely used technique:– Two programmers – one workstation– Programmer one is at the keyboard and have

focus on the details that are being typed.– Programmer two supervises programmer one’s

work and has the broader perspective and keeps focus on structure and what’s going to happen next.

– After a period (10 – 15 min.) programmer one and two swop roles.

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Exercise

• Do the Forest exercise (1 – 3) on Session04.docx.

• Use pair programming.

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Implementing n - m• A customer may have one or more

accounts, an account may belong to one or more customers.

• Possible solution: A collection of BankAccounts in Customer (accounts) and a collection of Customers (owners) in BankAccount.

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In the Code

public class BankAccount{ //…

private List<Customer> owners; //… public BankAccount(int no, double ir, double bal){ //… owners = new List<Customer>(); } public void AddOwner(Customer c) { owners.Add(c); }//…

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View Source

Exercise

• Do part 4 and 5 (Banking) of the exercises on Session04.docx

• Use pair programming.

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Implementing Associations Design Choices

• Several possibilities for the different cardinalities.

• The choice mostly depends on business logic (use cases)

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1 – 1:

• One of the objects must have a reference to the other.

• But which one?• Depends of business logic:

– Is access typically from Customer

– or

– from BankAccount?

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In Banking1

1 – n (1..*)

• Again – it depends on business logic:

– A collection of references on the 1-side?

– A single reference on the n-side?

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In Banking2

n - m (*..*)

• Here we have no choice:– There must be a collection in at least one of the classes:

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• But this is often a problematic solution:• Complicated to maintain.• Lots of coding is required for doing updates, inserts and deletes.

In Banking3

n - m (*..*)

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• But this is often a problematic solution:• Complicated to maintain.• Lots of coding is required for doing updates, inserts and deletes.

• Often it is a good design to introduce a connection object:

• And replace the n – m association by two 1 – n associations.• The designs for 1 – n may then be applied.• If there are any information on the association, this is necessary:

Example: Project Management

• Lets look at an other example:– An employee may work on several projects.– A project may have several employees working on it.– We need to record the number of hours a given

employee has spent on a given project:

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Let’s dive into the code

Exercises• Do part 6 (EmpProjV1) on Session04.docx

• Do part 7 (Banking3) on Exercises03.pdf.

• Use pair programming.

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