5. Introduction to
Classes
Objects & Classes
Import a class
BankAccount Class
Constructor
Copying Object
Reference
Objects & Classes
C is a structured
programming language
and its structures are
such as structs, unions,
and procedures.
Java, on the other
hand, consists of
methods, interfaces,
and a much more
highly developed class
structure.
The class keyword
introduces a class
definition and is
followed by the class
name.
The nameof the class
is called identifier.
Objects & Classes
Object is like a black
box which has calling
methods for doing
something
Object or black box
has part of code and
data
System.out is the
object of (Class)
PrintStream
Therefore, Class is like a
object's factory.
Class will define the details
of object's data and code
for object's method
Java Syntax for Object
Construction
new ClassName
(parameters)
When object is built by
syntax "new" it will
return the reference of
object
Example of Class :
Shape
Import a class
import
packageName.ClassName;
import java.awt.Rectangle;
// Class Rectangle is in
package named
java.awt
// (awt abbreviate
from Abstract
Windowing Toolkit)
For String and System
class are in package
named java.lang
which they do not
need to import
because it is import
automatically
We can import every
class in any package
by
import packagename.*;
import java.awt.*;
import java.io.*;
becareful!
import java.*.*; //
wrong
If we not import at the
start of program, we
should do like this:
java.awt.Rectangle
rect1;
rect1 = new
java.awt.Rectangle(10,
15, 20, 30);
Example
// Comparison.java
import javax.swing.
JOptionPane;
public class
Comparison {
public static void main
(String args[])
{
String firstnumber,
secondnumber.
int num1, num2;
firstnumber =
JOptionPane.
showInputDialog
(“Enger first integer”);
secondnumber =
JOptionPane.
showInputDialog
(“Enger second
integer”);
num1 =
Integer.parseInt
(firstnumber);
BankAccount Class
public class
BankAccount
{ private double
balance; // instance
variables
public void deposit
(double amount) //
method
{ balance = balance +
amount; }
public void withdraw
(double amount)
{balance = balance -
amount; }
public double
getBalance()
{ return balance; }
}
Constructors
A constructor is used
to initialize the
instance variables of
an object.
They have the same
name as their class.
They are always
invoked with the new
operator.
The new operator
allocates memory for
the object.
A default constructor
takes no parameters.
A general constructor
can take parameters.
Constructors
public class BankAccount
{ private double
balance;
public BankAccount() //
Default Constructors
{ balance = 0; }
public BankAccount
(double init) //
Constructors
{ balance = init; }
public void deposit
(double amount)
{ balance = balance +
amount; }
public void withdraw
(double amount)
{balance = balance -
amount; }
public double
getBalance()
{ return balance; }
}
How to use
BankAccount class
Once, we create a
class, we can create a
new object of this
class using new
command.
For example,
BankAccount myChecking
= new BankAccount();
Since we declare this
class as public, we can
call this class from
other classes.
How to use
BankAccount class
public class
BankAccountTest
{ public static void
main(String[] args)
{ BankAccount account
= new BankAccount
(10000);
final double INT_RATE
= 5;
double interest;
// compute and add
interest
interest =
account.getBalance() *
INT_RATE / 100;
account.deposit
(interest);
System.out.println
(“Balance after year 1
is”
+ account.getBalance()
);
// add interest again
interest =
account.getBalance() *
INT_RATE / 100;
account.deposit
(interest);
}
}
Copying Numbers
double balance1 =
1000;
double balance2 =
balance1; // figure (a)
balance2 = balance2 +
500; // figure (b)
Copying Object
References
BankAccount account1
= new BankAccount
(1000);
BankAccount account2
= account1;
account2.deposit(500);
null Reference
BankAccount account1
= new BankAccount
(1000);
BankAccount account2
= account1;
account1 = null; // an
object variable that
refers to no object.
String greeting =
“Hello”;
String message = “”; //
an empty string
String comment =
null; // refers to no
string at all
int g = greeting.length
(); // return 5
int m =
message.length(); //
return 0
int c = comment.length
(); // program
terminate
Thursday, June 30, 2011
lesson 5-Introduction to Classes
11:07 AM
Ravi mishra
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