Chapter 3 Writing classes

Writing Classes
Writing Classes
Chapter 3
Chapter 3
1
Out lin e
•
Introduction to Object-Oriented Programming (OOP)
•
Writing a Java class
•
Encapsulation and accessor methods
2
Procedural Programming
Procedural Programming


and
and
Object-Oriented Programming
Object-Oriented Programming

Procedural Programming consists of designing a set of
functions (or algorithms) to solve a problem
•
Algorithms + Data Structures = Programs

Object-Oriented Programming (OOP) puts data first, then
looks at the algorithms that operate on the data
3
Concepts in OOP

Concepts in OOP

Class
•
A class is the model, pattern, or blueprint from which
objects are actually made
•
A class means a category of things
•
A class can be used in Java as the data type

Object
•
Object means a particular item that belongs to a class
•
Object also called an “instance”

Example:
•
String s1 = "Hello";
Here, String is the class, and the variable s1 is objects (or
“instances of the String class”)
4
Concepts in OOP
Concepts in OOP
(cont.)
(cont.)

Attributes
•

The properties that describe an object

Behaviors
•
Activities of an object

Each object has a set of attributes (state) and a set of
behaviors
5
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Example: Class and Object
Class: Ball
Attributes: radius,
color
Behavior: thrown,
bounced, rolled
Object1
radius = 5
color = red
thrown,
bounced,
rolled
Object3
radius = 3
color = green
thrown,
bounced,
rolled
Object2
radius = 10

color = blue
thrown,
bounced,
rolled
6
Examples
Examples

List attributes and behaviors of students
•
Student code, name, gender, birthday, birthplace, class,
grade 1, grade 2,…
•
Calculate average grade, view grade, pay tuition,…

Suppose that you write a program to calculate area and
perimeter of a rectangle. Find objects and list attributes and
behaviors of them
•
Object: Rectangle
•
Attributes: width, height
•
Behaviors: Calculate area, calculator perimeter
7
Examples
Examples


(cont.)

(cont.)

Suppose that you want to write a program to monthly pay the
employees of a company. Paying employees based on
products they made, in addition, you have to compute Social
Security and Medicare taxes. Find objects and list attributes
and behaviors of them
•
Object: Employee
•
Attributes: employee number, name, address, social
security number, products,…
•
Behaviors: compute employee pay, compute employee
Social Security and Medicare taxes, mail employee a
paycheck once a month,…
8
Objects model
Objects model
ClassName
- Attributes
+ Behaviors
Properties
Data
Fields
Variables
Instance data
Methods
9
Objects model:

Objects model:
Example
Example
Rectangle
-
width
-
height
+ Calculate area
+ Calculator perimeter
Student
-
Student code
-
Name
-
Gender
-
Birthday
-
Birthplace
-
Class
-
Grade 1
-
Grade 2
+ Calculate average
+ View grade
+ Pay tuition

10
Out lin e
•
Introduction to Object-Oriented Programming (OOP)
•
Writing a Java class
•
Encapsulation and accessor methods
11
Writing a Java Class
Writing a Java Class
class ClassName
{
property1
property2
. . .
constructor1
constructor2
. . .
method1
method2
. . .
}
12
Writing class Rectangle
Writing class Rectangle
class ClassName
{
property1
property2

. . .
constructor1
constructor2
. . .
method1
method2
. . .
}
class Rectangle
{
double width;
double height;

public double getArea()
{
return width*height;
}

public double getPerimeter()
{
return (width+height)*2;
}
}
Rectangle
-
width: double
-
height: double
+ getArea() : double
+ getPerimeter() : double

13
Writing class Student
Writing class Student
class Student{
String studentCode, name, birthplace,
classID;
boolean gender;
Date birthday;
double grade1, grade2;
public double getAverage() {
return (grade1+ grade2)/2;
}
public void viewGrade() {
System.out.println( studentCode + “\t”+
name + ”\t”+ birthday + ”\t”+ birthplace + ”\t”+
classID + ”\t”+ getAverage() );
}
}
class NameOfClass
{
attribute1
attribute2
. . .
constructor1
constructor2
. . .
method1
method2
. . .
}

Student
-
studentCode: String
-
name: String
-
gender: boolean
-
birthday: Date
-
birthplace: String
-
classID: String
-
grade1: double
-
grade2: double
+ getAverage(): double
+ viewGrade(): void
14
Declaring objects
Declaring objects

Once a class is defined, you can declare variables (object
reference) of that type
Rectangle r1, r2;
Student st;

Object references are initially null
•

The null value is a distinct type in Java and is not equal to
zero

To explicitly create the object, you must use the new operator
Rectangle r = new Rectangle();
st = new Student();
ClassName objectName = new ClassName();
15
Accessing instance variables
Accessing instance variables

To access fields and methods of an object, using the dot
operator
objectName.fieldName
objectName.methodName(argumentsToMethod)

Example:
r.width = 10;
r.height = 5;
double x= r.getArea();
double y= r.getPerimeter();
16
Example: Writing class Rectangle (1)
Example: Writing class Rectangle (1)
public class Rectangle
{
double width, height;

public double getArea()
{

return width * height;
}
public double getPerimeter()
{
return (width+height) * 2;
}
}
Rectangle.java
public class RectangleTest
{
public static void main(String[] args) {
Rectangle r = new Rectangle();
r.width = 10;
r.height = 5;
double area = r.getArea();
double peri = r.getPerimeter();
System.out.println("Area is "+ area);
System.out.println("Perimeter is "+ peri);
}
}
RectangleTest.java
Note: You almost always make instance variables private
and use methods to access them (see next content…)
In which case outside code can’t access them directly
17
Constructor
Constructor

Constructor is a special method called when a class is created
with new

•
Constructor is especially useful for supplying values of
fields
•
Constructors are declared through:
•
Notice that the constructor name must exactly match the
class name
•
Constructors have no return type (not even void)
•
A class can have multiple constructors
public ClassName (agrsList) {
…
}
18
Constructor
Constructor
(cont.)
(cont.)

Java automatically provides a zero-argument constructor
(default constructor) if and only if the class doesn’t define it’s
own constructor
•
That’s why you could say
Rectangle r = new Rectangle(); even though a constructor
was never defined

This default constructor sets all the instance fields to their

default values
•
All numeric data contained in the instance fields would be
0, all booleans would be false, and all object variables
would be set to null

If you have your own constructor, you should provide a default
constructor, it can do nothing
19
Example: Writing class Rectangle (2)
Example: Writing class Rectangle (2)
public class Rectangle
{
double width, height;
public Rectangle( double x,
double y )
{
width = x;
height = y;
}
public double getArea() {
return width * height;
}
public double getPerimeter() {
return (width+height) * 2;
}
}
Rectangle.java
public class RectangleTest
{

public static void main(String[] args) {
Rectangle r = new Rectangle(10,5);
double area = r.getArea();
double peri = r.getPerimeter();
System.out.println("Area is "+ area);
System.out.println("Perimeter is "+ peri);
}
}
RectangleTest.java
Can write:
Rectangle r = new Rectangle(); ?
Class with Constructors
The
The
this
this
variable
variable

The this object reference can be used inside any non-static
method to refer to the current object

The common uses of the this reference are:
•
To pass a reference to the current object as a parameter to
other methods:
someMethod(this);
•
To resolve name conflicts
–

Using this permits the use of instance variables in methods
that have local variables with the same name
•
To call another constructor
–
In a class, a constructor can invoke other constructor, by
using this(agrs), note the this statement must appear
as the first line of code in the constructor (otherwise, a
compiler error will occur)
21
Example: Writing class Rectangle (3)
Example: Writing class Rectangle (3)
public class Rectangle
{
double width, height;
public Rectangle( double width,
double height )
{
this.width = width;
this.height = height;
}
public Rectangle()
{
this(0, 0);
}
public double getArea() {
return width * height;
}
public double getPerimeter() {
return (width+height) * 2;

}
}
Rectangle.java
public class RectangleTest
{
public static void main(String[] args) {
Rectangle r = new Rectangle();
double area = r.getArea();
double peri = r.getPerimeter();
System.out.println("Area is "+ area);
System.out.println("Perimeter is "+ peri);
r = new Rectangle(10, 5);
area = r.getArea();
peri = r.getPerimeter();
System.out.println("Area is "+ area);
System.out.println("Perimeter is "+ peri);
}
}
RectangleTest.java
Using this
22
Out lin e
•
Introduction to Object-Oriented Programming (OOP)
•
Writing a Java class
•
Encapsulation and accessor methods
23


Encapsulation is the technique of making the fields in
a class private and providing access to the
fields via public methods

Encapsulation is also referred to as data hiding

Encapsulation is one of the four fundamental OOP concepts.
The other three are inheritance, polymorphism, and
abstraction
Encapsulation
Encapsulation
Client (user)
Client (user)
Methods
Data
24
Encapsulation
Encapsulation
(cont.)
(cont.)

Instance variables should always be private
•
private variables can be accessed only by the methods
of the class
•
Providing accessor methods via public methods: get/set

Methods can be defined with public or private
•

public method: provide services to the client of the class
so that they can be invoked by the client  service
methods
•
private method: cannot be invoked from outside the
class. The only purpose of a private method is to help the
other methods of the class do their job  support methods
25