What is instance members? Explain with an example
Instance members are the attributes and methods of a class
They share one copy each per object
Since an object is an instance of a class, memory has been set aside for each object to contain its own copies of instance members.
Instance variables and instance methods are referred by joining object reference variable and the name of the instance variable or name of the method.
Examplestudent.studId – an instance variable of student object is referred
student.computeAverage() – an instance method of student object will be invoked
What is instance variable? Explain with an example
1. An instance variable is defined in a class scope
2. Each object of the class has a separate copy of the instance variable
3. Instance variables values are unique to each instance of a class
4. If there are 3 copies of a class Product, there are 3 copies of each instance variable defined in the class
5. Instance variables can be accessed directly by the object reference variable
Exampleclass Car
{
String make,fuel;
int year;
. . . .
}
// Create object of Car
Car myCar = new Car();
myCar.make = “Honda”;
myCar.model=”Diesel”;
myCar.year = 2009;
6. The instance variables make, model and year are accessed by the reference variable ‘myCar’
What is instance method? Explain with an example
Instance methods are belong to the object
1. Instance methods are invoked by the reference variables of the object
2. Instance methods are unique to each instance of a class
3. Instance methods can be accessed directly by the object reference variable
Exampleclass Car
{
String make, fuel;
int year;
void computeServiceCharges()
{
. . . .
}
void computeSparesAmount()
{
. . . .
}
}
// Create object of Car
Car myCar = new Car();
myCar.computeServiceCharges();
myCar.computeSparesAmount();
The instance methods computeServiceCharges() and computeSparesAmount() are accessed by the reference variable ‘myCar’.
What is static member? Explain with an example
Static member belongs to class
Each static member will have exactly one copy per class
Static members are accessible by their class name
Example Integer.parseInt(), Float.parseFloat(), DriverManager.getConnection() etc.
All methods in java.lang.Math class are static methods
Example Math.sqr(), Math.log(), Math.abs() etc.
Static methods can not use any instance variables
The keyword ‘this’ can not be used in a static method
The following
example illustrates static members:
public class AccessStaticMethod
{
public static void staticMethod()
{
System.out.println(“I am in staticMethod()” );
}
public static void main(String[] args)
{
num1 = 30;
num2 = 23;
staticMethod(); // static method can invoke another static method
}
}
What is static variable? Explain with an example
Static variables are belongs to the class as a whole
The static variables are allocated in the memory when the class is loaded
Static variables are accessible by their class name
Example Integer.MIN_VALUE, Math.PI etc.
Static variables can not use any instance variables
The keyword ‘this’ can not be used with static variable
The following
example illustrates static variables:
import java.util.Date;
class MyClass
{
static Date date new Date();
Date date2 new Date();
public static void main(String[] args)
{
//Display static variable
System.out.println(“ Static variable” );
System.out.println(MyClass.date);
//Delay for five seconds
try
{
Thread.currentThread().sleep(5000);
}
catch(InterruptedException e){}
}
}
