Chapter 7 Objects and Classes 1 Fall 2012 CS2302: Programming Principles.

1

Chapter 7 Objects and Classes

Fall 2012 CS2302: Programming Principles

Content

Classes and Objects How to define classes Reference Type and default value Using classes from the java library Visibility Modifiers Data Field Encapsulation Static variables

2 Fall 2012 CS2302: Programming Principles

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Classes and Objects

A Java program consists of one or more classes

A class is an abstract description of objects Here is an example class:

– class Dog { ...description of a dog goes here... }

Here are some objects of that class:


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More Objects Here is another example of a class:

– class Window { ... } Here are some examples of Windows:


OO Programming Concepts

Object-oriented programming (OOP) involves programming using objects. – A class is a piece of the program’s source

code that describes a particular type of objects.

– An object is called an instance of a class. A program can create and use more than one object (instance) of the same class.


Class Object A blueprint for

objects of a particular type

Defines the structure (number, types) of the attributes

Defines available behaviors of its objects

Attributes

Behaviors


Class: Car Object: a car

Attributes: String model Color color int numPassengers double amountOfGas

Behaviors: Add/remove a passenger Get the tank filled Report when out of gas

Attributes: model = "Mustang" color = Color.YELLOW numPassengers = 0 amountOfGas = 16.5

Behaviors:


Class– User-defined data type

Object– Instance of class, A role of variable – Define a correspond class to define a object

Define a class

[modifier] class ClassName {// field declarations

//Constructors // method declarations

}

Defining Classes for Objects

Describe the structure of object

Data Field(variable, constant)

Define actions of object

Method Field

public, final,

abstract


Constructors

Short procedures for creating objects of a class

Always have the same name as the class

Initialize the object’s fields

May take parameters, but no returns

A class may have several constructors that differ in the number and/or types of their parameters


Class Example class Circle {

/** The radius of this circle */ double radius = 1.0; /** Construct a circle object */ Circle() { } /** Construct a circle object */ Circle(double newRadius) { radius = newRadius; } /** Return the area of this circle */ double getArea() { return radius * radius * 3.14159; }

}

Data field

Method

Constructors


Object Example Public class TestCircle {

/** Main method */ Public static void main(String[] args){ /** Create a circle with radius 1.0*/ Circle circle1 = new Circle(); System.out.println(“The area of the Circle is ” + circle1.getArea()); /** Create a circle with radius 25*/ Circle circle2 = new Circle(25); System.out.println(“The area of the Circle is ” + circle2.getArea()); /** Modify circle radius */ circle2.radius = 100; System.out.println(“The area of the Circle is ” + circle2.getArea());

}

Access Method

Create Object

Access Data


Class vs. Object A piece of the program’s source code Written by a programmer

An entity in a running program

Created when the program is running (by the main method or a constructor or another method)


Class vs. Object

Specifies the structure (the number and types) of its objects’ attributes — the same for all of its objects

Specifies the possible behaviors of its objects

Holds specific values of attributes; these values can change while the program is running

Behaves appropriately when called upon


Diagram of program structure

A program consists of one or more classes

Typically, each class is in a separate .java file

Program

File File

File

File

ClassVariables

Constructors

Methods

Variables

Variables

Statements

Statements


Object type is a Reference Type

Go over the textbook slides from page 29 to 38


Encapsulation

Encapsulation is one of the four fundamental OOP concepts. The other three are inheritance (is the capability of a class to use

the properties and methods of another class), polymorphism (more than

one form), and abstraction (simplifying complex reality by modeling

classes ).

Encapsulation can be described as a protective barrier that prevents the code and data being randomly accessed by other code defined outside the class. Access to the data and code is

tightly controlled by an interface.

Encapsulation provides a technique of making the fields in a class private and providing access to the fields via public

methods. 16 Fall 2012 CS2302: Programming Principles

The fields of a class can be made read-only or write-only.

A class can have total control over what is stored

in its fields.

The users of a class do not know (like a blackbox)

how the class stores its data. A class can change

the data type of a field, and users of the class do

not need to change any of their code.

Benefits of Encapsulation


Encapsulation Implementation

If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. For this reason, encapsulation is also referred to as data hiding.

A class interacts with other classes only through constructors and public methods

Other classes do not need to know the mechanics (implementation details) of a class to use it effectively


example of Encapsulation

http://www.tutorialspoint.com/java/java_encapsulation.htm

Visibility Modifiers

We accomplish encapsulation through the appropriate use of visibility modifiers

Visibility modifiers specify which parts of the program may see and use any particular class/method/field

A modifier is a Java reserved word that specifies particular characteristics of a programming construct

We've used the modifier final to define a constant Java has three visibility modifiers: public, private, and protected

We will discuss the protected modifier later in the course


Visibility Modifiers - Classes

A class can be defined either with the public modifier or without a visibility modifier.

If a class is declared as public it can be used by any other class

If a class is declared without a visibility modifier it has a default visibility. The default modifier restricts access to the same package.

Classes that define a new type of objects, that are supposed to be used anywhere, should be declared public.


Visibility Modifiers - Members

A member is a field, a method or a constructor of the class.

Members of a class can be declared as private, protected, public or without a visibility modifier:

private int hours; int hours; public int hours;


Public Visibility

Members that are declared as public can be accessed from any class that can access the class of the member

Example: the methods getHours(), secondElapsed() and setTime() are part of the interface of class Clock so we define them as public.

We do not want to reveal the internal representation of the object’s data. So we usually declare its state variables as private (encapsulation).


Private Visibility

A class member that is declared as private, can be accessed only by code that is within the class of this member.

We hide the internal implementation of the class by declaring its state variables and auxiliary methods as private.

Data hiding is essential for encapsulation.


Illegal Access - example

// Example of illegal accessclass BankAccountTest { public static void main(String[] args) { BankAccount victim = new BankAccount(2398742); victim.balance = victim.balance - 500; // this will not compile! }}

public class BankAccount { private long accountNumber; private double balance; // …


Encapsulation - example

public void transfer(double amount, BankAccount targetAccount) { withdraw(amount); targetAccount.deposit(amount);}

// alternative version (valid, but not so nice)public void transfer(double amount, BankAccount targetAccount) { balance -= amount; targetAccount.balance += amount;}


Getters/Accessors and Setters/Mutators Methods

class Clock { Private String time;

void setTime (String t) {time = t;} String getTime() {return time;}

} class ClockTestDrive {

public static void main (String [] args){ Cock c = new Clock();

c.setTime("12345") String tod = c.getTime();

System.out.println(time: " + tod); } }27 Fall 2012 CS2302: Programming Principles

Encapsulation

Constructors and methods can call other public and private methods of the same class.

Constructors and methods can call only public methods of another class.

Class X private field

private method

Class Y

public method public method


The Static Modifier

The static modifier can be applied to variables or methods.

It associates a variable or method with the class rather than an object.

Methods that are declared as static do not act upon any particular object. They just encapsulate a given task, a given algorithm.


Static Variables

It is a variable which belongs to the class and not to object (instance) --- class variable

A single copy to be shared by all instances of the class

A static variable can be accessed directly by the class name and doesn’t need any object– Syntax : <class-name>.<variable-name>


Static Methods

It is a method which belongs to the class and not to the object(instance)

A static method can access only static data. It can not access non-static data (instance variables)

A static method can call only other static methods and can not call a non-static method from it.

A static method can be accessed directly by the class name and doesn’t need any object– Syntax : <class-name>.<method-name>

A static method cannot refer to “this” or “super” keywords in anyway


Static Variables - Example

public class BankAccount {

private long accountNumber; private double balance; private static int numberOfAccounts = 0;

public BankAccount() { this.accountNumber = ++numberOfAccounts; this.balance = 0; } public static int getNumberOfAccounts { return numberOfAccounts; }}


Summary

A program consists of one or more classes A class is a description of a kind of object

– In most cases, it is the objects that do the actual work A class describes data, constructors, and

methods– An object’s data is information about that object– An object’s methods describe how the object behaves– A constructor is used to create objects of the class

Methods (and constructors) may contain temporary data and statements (commands)


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Chapter 7 Objects and Classes 1 Fall 2012 CS2302: Programming Principles.

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