Java-Variables_about_different_Scope.ppt

Scope of Java Variables

2. ●This module is intend to develop basic skills in
working with different types of variables that java
supports.
●Clearly differenciate the advantages and
disadvantages of working with different types of
variables.
Module Overview
2

3. Use different types of variables in a program
Participant should be able to identify the type of variable
to use according to the program needs.
Understand between static and non-static variables.
Undestand the memory allocations to different variables
in java.
Understand the working of primitive variable and reference
variable
Declare variables of class type
Construct an object using new
Describe default initialization
Describe the significance of a reference variable
State the consequences of assigning variables of class type
Objectives
3

4. Variable
Variable:A variable refers to a memory location with some value.
These memory locations are identified by identifiers. Amount of
memory allocation to the variables are decided by the data type
of the variable.
Syntax:
type identifier [ = value] ;
Example:
int weight=40;
float salary=22222.0f
String name = “Taksheel”
4

5. Java supports 3 types of variables .
Local variables
Instance variables
Class/static variables
Types of Variables
5

6. Local variables :
Local variables are declared in methods,
constructors, or blocks.
Local variables are created when the method,
constructor or block is entered and the variable
will be destroyed once it exits the method,
constructor or block.
Access modifiers cannot be used for local
variables.
Local variables are visible only within the
declared method, constructor or block.
Local Variables

7. Instance variables are declared in a class, but
outside a method, constructor or any block.
Memory is allocated to them when object is
created and initialised with default values or
literals values.
Instance variables are accessed directly by
methods, constructors, blocks, or essential
parts of an object.
Instance variables can be declared in class
level which are called as static variables.
Instance Variables
7

8. Class variables also known as static variables are
declared with the static keyword in a class, but
outside a method, constructor or a block.
There would be only one copy ofstatic variable across
all objects of a single class.
Static variables are stored in static memory. Normally
static variables are declared public /private and
final.Which then function as constants.
Class or Static Variables
<<Module name>>/<<Session #>>/<<Lesson Name>> >>
8

9. • Static variables are created when the program starts
and destroyed when the program stops.
• Visibility and Default values are similar to instance
variables. Static values can be assigned in static
initializer blocks.
• Static variables can be accessed by calling with the
class name . ClassName.VariableName.
Class or Static Variables(Cont…)
<<Module name>>/<<Session #>>/<<Lesson Name>> >>
9

10. Example Local Variable
public class Test{
public void machineAge(){
int age = 0;//local variable
age = age + 7;
System.out.println(“Machine age is : " + age)
}

public static void main(String args[]){
Test test = new Test();//test is an instance of Test class
Test.machineAge();
}
}
10

11. Example Instance Variable
<<Module name>>/<<Session #>>/<<Lesson Name>> >>
11
public class Machine{
String mname;//instance variable
float mcost;//instance variable
public float machineCost(){
float vat= 7/100;//local variable
this.mcost= mcost + vat;
System.out.println(“Machine age is : " + age)
}
public static void main(String args[]){
Machine sewing = new Machine();//machine is an instance of
Machine class
System.out.println(“Machine Cost:”+ sewing.machineCost());
}
}

12. Example Static Variable
public class Machine{
String mname;//instance variable
float mcost;//instance variable
static String manu_name;//static variable
public float machineCost(){
float vat= 7/100;//local variable
this.mcost= mcost + vat;
System.out.println(“Machine age is : " + age)
}
}
<<Module name>>/<<Session #>>/<<Lesson Name>> >>
12

13. Static Variable(Cont…)
Class MachineApps{
public static void main(String args[]){
Machine sewing M= new Machine();//sewing is an instance of
Machine class
Machine.manu_name=“Usha”;
System.out.println(“Machine Cost:”+ sewingM.machineCost());
Machine fan= new Machine();/
System.out.println(“Machine manu Name:”+
sewingM.manu_name);
System.out.println(“Machine manu Name:”+ fan. manu_name);
}
}
<<Module name>>/<<Session #>>/<<Lesson Name>> >>
13

14. Scope Example
public class ScopeExample {
private int i=1;
public void firstMethod() {
int i=4, j=5;
this.i = i + j;
secondMethod(7);
}

15. Scope Example(Cont…)
public void secondMethod(int i) {
int j=8;
this.i = i + j;
}
}
public class TestScoping {
public static void main(String[] args) {
ScopeExample scope = new ScopeExample();
scope.firstMethod();
}
}

16. Allocation of Memory
Variables i,j are
local variables
which have
different values
in methods.
These variables
are declared in
method stack.
InstanceVariable i
is declared in
heap.

17. Thank You