1. Tutorial 6 February 4th/5th, 2015
CPSC 233
Tutorial 6
February 4th/5th, 2015

2. Defining Classes and Mothods
Java is a true OO language and therefore the underlying structure of all Java programs is classes.
Anything we wish to represent in Java must be encapsulated in a class that defines the “state” and “behaviour” of the basic program components known as objects.

3. Classes
A class is a collection of fields (data) and methods (procedure or function) that operate on that data.
Circle
centre
radius
circumference()
area()

4. Creating objects/instances of a class
Objects are created dynamically using the new keyword.
aCircle and bCircle refer to Circle objects
aCircle = new Circle() ;
bCircle = new Circle() ;

5. Executing Methods in Object/Circle
Using Object Methods:
sent ‘message’ to aCircle
Circle aCircle = new Circle();
double area;
aCircle.r = 1.0;
area = aCircle.area();

6. Constructors
Circle(double r) { radius = r; } Circle() { radius = 1.0; myCircle = new Circle(5.0);
Constructors are a special kind of methods that are invoked to construct objects.

7. Constructors, cont.
A constructor with no parameters is referred to as a default constructor.
Constructors must have the same name as the class itself.
Constructors do not have a return type—not even void.
Constructors are invoked using the new operator when an object is created. Constructors play the role of initializing objects.

8. Visibility Modifiers and Accessor Methods
public
The class, data, or method is visible to any class in any package.
private
The data or methods can be accessed only by the declaring class.
The get and set methods are used to read and modify private properties

9. Exercise 1
Define a class called Counter. An object of this class is used to count things, so it records a count that is a nonnegative whole number.

10. Exercise 1
Include methods to:
set the counter to zero
to increase the count by 1
and to decrease the count by 1
include an accessor method that returns the current count value
include a method that outputs the count to the screen

11. Exercise 1
Be sure that no method allows the value of the counter to become negative.
The only method that can set the counter is the one that sets it to zero.
Also, write a program to test your class definition.

12. Counter Class

13. CounterTest

14. Exercise 2
Consider a class PersonAddress that represents an entry in an address book. Its attributes are:      * The first name of the person      * The Last name of the person      * The address of the person      * The telephone number of the person  It will have methods to:      * Access each attribute      * Change the address      * Change the telephone number      * Test whether two instances are equal based solely on name     1. Write a method heading for each method.     2. Write preconditions and postconditions for each method.     3. Write some Java statements that test the class.     4. Implement the class.

15. PersonAddress Class

16. PersonAddress Test