1. Object Oriented Programming
Lecture 11
Object Oriented Programming
Richard Gesick
Figures from Lewis, “C# Software Solutions”, Addison Wesley

2. Topics Software Development Identifying Classes & Objects
Static Members
Class Relationships
“this”
Parameters Revisited
Overloading
Methods
Operators
Testing

3. The Software Life Cycle
The overall life cycle of a program includes use and maintenance:
A version of the software that is made available to user is called a release
Use
Development
Maintenance

4. Development vs. Maintenance
Use and
Maintenance

5. The Waterfall Model Establish requirements Create design Implement
code
Test
system

6. An Iterative Development Process
Establish
requirements
Create
design
Implement
code
Test
system

7. Identifying Classes & Objects
Identify potential classes within the specification
Nouns

9. In Gaming… How do you identify the classes within a game?
What have you seen so far in labs and assignments?

10. What Goes in the Class How do you decide what should be in the class
Data
Methods
Not easy, but must be done

11. Static Members Static methods can be invoked via the class name
Static variables are stored at the class level (one copy for all instances)

12. The static Modifier
Remember that static methods (also called class methods) that can be invoked through the class name rather than through a particular object
For example, the methods of the Math class are static:
Math.sqrt (25)
To write a static method, we apply the static modifier to the method definition
The static modifier can be applied to variables as well
It associates a variable or method with the class rather than with an object

13. Static Variables Static variables are also called class variables
Normally, each object has its own data space, but if a variable is declared as static, only one copy of the variable exists
private static int count;
Memory space for a static variable is created when the class in which it is declared is loaded
All objects created from the class share static variables
Changing the value of a static variable in one object changes it for all others
Local variables cannot be static

14. Static Methods class Helper public static int triple (int num) {
int result;
result = num * 3;
return result; }
class Helper
Because it is static, the method can be invoked as:
value = Helper.triple (5);

15. Static Methods
Static methods cannot reference instance variables, because instance variables don't exist until an object exists
However, a static method can reference static variables or local variables

16. Class Relationships
Classes can have various relationships to one another. Most common are:
Dependency (“uses”)
Aggregation (“has a”)
Inheritance (“is a”)

17. Object Relationships
Objects can have various types of relationships to each other
A general association, as we've seen in UML diagrams, is sometimes referred to as a use relationship
A general association indicates that one object (or class) uses or refers to another object (or class) in some way
We could even annotate an association line in a UML diagram to indicate the nature of the relationship
Author
Book
writes

18. Dependency One class dependent (uses) another class
Game uses ball, paddle
Ship uses bullet
Sometimes, a class depends on another instance of itself
Is one date equal to another date?
Is one picture equal to another picture?

19. Rational Class Design
Let’s design a class that represents a rational number (fraction)
What data needs to be stored?
What methods or operations are needed?
Look at the following client code:

20. Rational r1 = new Rational(6, 8);
Rational r3, r4, r5, r6, r7;
Console.Out.WriteLine("First rational number: " + r1);
Console.Out.WriteLine("Second rational number: " + r2);
if (r1.Equals(r2))
Console.Out.WriteLine("r1 and r2 are equal.");
else
Console.Out.WriteLine("r1 and r2 are NOT equal.");
r3 = r1.Reciprocal();
Console.Out.WriteLine("The reciprocal of r1 is: " + r3);
r4 = r1.Add(r2);
r5 = r1.Subtract(r2);
r6 = r1.Multiply(r2);
r7 = r1.Divide(r2);
Console.Out.WriteLine("r1 + r2: " + r4);
Console.Out.WriteLine("r1 - r2: " + r5);
Console.Out.WriteLine("r1 * r2: " + r6);
Console.Out.WriteLine("r1 / r2: " + r7);
Console.In.ReadLine(); // Wait for enter key

22. UML Design
Rational

23. Constructor
public Rational (int numer, int denom) { }

24. Reciprocal
public Rational Reciprocal ( ) { }

25. ToString
public override string ToString ( ) { }

26. Reduce
private void Reduce ( ) { if (numerator != 0) int common = Gcd(Math.Abs(numerator), denominator); numerator = numerator / common; denominator = denominator / common; }

27. Gcd
private int Gcd (int num1, int num2) { while (num1 != num2) if (num1 > num2) num1 = num1 - num2; else num2 = num2 - num1; return num1; }

28. Add
public Rational Add (Rational op2) { }

29. Subtract
public Rational Subtract (Rational op2) { }

30. Multiply
public Rational Multiply (Rational op2) { }

31. Divide
public Rational Divide (Rational op2) { }

32. Equals
public bool Equals (Rational op2) { }

33. Aggregation One class is “made up” of other classes
“has a” relationship
Gameboard has a marble
Deck has a card

34. Aggregation
An aggregate object is an object that contains references to other objects
For example, an Account object contains a reference to a String object (the owner's name)
An aggregate object represents a has-a relationship
A bank account has a name
Likewise, a student may have one or more addresses

35. Aggregation in UML
An aggregation association is shown in a UML class diagram using an open diamond at the aggregate end
StudentBody
+ Main (args : String[]) : void
+ ToString() : String 1 2
Student
- firstName : String
- lastName : String
- homeAddress : Address
- schoolAddress : Address
- streetAddress : String
- city : String
- state : String
- zipCode : long
Address

36. The this Reference
The this reference allows an object to refer to itself
That is, the this reference, used inside a method, refers to the object through which the method is being executed
Suppose the this reference is used in a method called tryMe
If tryMe is invoked as follows, the this reference refers to obj1:
obj1.tryMe();
But in this case, the this reference refers to obj2:
obj2.tryMe();

37. The this reference
The this reference can also be used to distinguish the parameters of a constructor from the corresponding instance variables with the same names
public Account (String name, long acctNumber,
double balance) {
this.name = name;
this.acctNumber = acctNumber;
this.balance = balance; }

38. Assignment Revisited
The act of assignment takes a copy of a value and stores it in a variable
For primitive types:
num2 = num1;
Before
num1 5
num2 12
After
num1 5
num2

39. Reference Assignment Before After
For object references, assignment copies the memory location:
bishop2 = bishop1;
Before
bishop1
bishop2
After
bishop1
bishop2

40. Aliases
Two or more references that refer to the same object are called aliases of each other
One object (and its data) can be accessed using different reference variables
Aliases can be useful, but should be managed carefully
Changing the object’s state (its variables) through one reference changes it for all of its aliases