Chapter 3 Writing classes

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

Writing Classes

Chapter 3

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• Encapsulation and accessor methods

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Procedural Programming and

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

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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”

• String s1 = "Hello";

Here, String is the class, and the variable s1 is objects (or

“instances of the String class”)

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Concepts in OOP (cont.)

 2003 Prentice Hall, Inc All rights reserved.

Example: Class and Object

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 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

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• 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,…

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Fields Variables Instance data

Methods

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Objects model: Example

• Encapsulation and accessor methods

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Writing a Java Class

Writing 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

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Writing class Student

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() );

} }

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();

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Accessing instance variables

 To access fields and methods of an object, using the dot

operator

objectName.fieldName objectName.methodName(argumentsToMethod)

Example: Writing class Rectangle (1)

public class Rectangle

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

and use methods to access them (see next content…)

In which case outside code can’t access them directly 17

• 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

… }

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Constructor (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

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Example: Writing class Rectangle (2)

public class Rectangle

{

double width, height;

public Rectangle( doub le x,

double y ) {

width = x;

height = y;

}

public double getArea() {

return width * height;

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 this variable

 The this object reference can be used inside any non-static method to refer to the current object

• 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

Example: Writing class Rectangle (3)

public class Rectangle

{

double width, height;

public Rectangle( double width,

double height ) {

public double getArea() {

return width * height;

Rectangle r = new Rectangle();

double area = r.getArea();

double peri = r.getPerimeter();

}

RectangleTest.java

Using this

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 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

Encapsulation (cont.)

• private variables can be accessed only by the methods

of the class

• Providing accessor methods via public methods: get/set

• 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

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Example: Writing class Rectangle (4)

public class Rectangle

{

private double width, height;

public Rectangle (double width,

double height) {

} public void setHeight(double height) { this.height = height;

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

} public double getPerimeter() { return (width+height) * 2;

} }

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Example: Writing class Rectangle (4)

{ public static void main( String [] args ) {

System.out.println("Area is "+ area);

System.out.println("Perimeter is "+ peri);

} }

RectangleTest.java

Write accessor methods

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More details on Getters and Setters

 There need not be both getters and setters

• It is common to have fields that can be set at instantiation,

but never changed again (immutable field)

 Getter/setter names need not correspond to instance variable names

public class Ship {

public Ship(…) {

shipName = …;

} public String getShipName() { return shipName; }

// No setShipName method

}

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Note: Java naming conventions

 Start classes with uppercase letters

 Start other things with lowercase letters

• Instance variables, local variables, methods, parameters to

methods

public class MyClass {

public String firstName, lastName;

public String fullName() {

String name = firstName + " " + lastName;

return name;

} }

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Class design hints

 Always keep data private

 Always initialize data

 Don't use too many basic types in a class

• Example: Customer class

private String customerID;

private String customerName;

private String street;

private String city;

private String state;

private int zip;

 Not all fields need individual field accessors and mutators

 Make the names of your classes and methods reflect their

responsibilities

30 Create an Address class

 Syntax to create a class in Java?

 What is constructor? How many constructors can we have in a class?

 Why to make all instance variables private?

 How to access private instance variables?

 Write a class to calculate area of a circle with radius Test this class

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Read more

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