Difference between revisions of "Java/C3/Static-Variables/English"

Learning Objectives

• What are static variables
• Creating static variables and
• How to use static variables

Software Requirements

• Ubuntu 12.04
• JDK 1.7 and
• Eclipse 4.3.1

Prerequisites

You should also have the knowledge of classes, objects and instance variables in Java.

If not, for relevant Java tutorials, please visit the link shown.

What is a static variable?

It is also called a class variable.

It is declared using the static keyword.

Static Variable-Example

(Proceed with the Slide)

In this tutorial we will see it in detail.

Let us illustrate the usage of static variables with an example.

Consider a class created to represent student enrollments in an organization.

This class contains the Name, Id , Branch and Total Count of students enrolled.

Now let us visualize what happens when student enrollments are done.

By default, Total Count is 0.

The Name of the first student is ADIL.

Id is IT101. Branch is IT.

Now the Total Count is updated as 1.

Similarly when the second student AMAL gets enrolled, Total Count is updated to 2.

When the third student CAROL gets enrolled, Total Count is updated to 3.

Now we can identify that the variable Total Count is common to all the objects and has a single value.

So the variable Total Count can be represented as a static variable.

We can also see that the variables Name, Id and Branch have their own copies for each object.

Also they have specific values for each object.

So these variables can be treated as instance variables.

Instance Vs Static Variable

Every object has its own copies for instance variables.

Id, Name, Branch are examples of instance variables.

Static variables in a class share a single

copy with all objects in that class.

Total Count is an example of static variable.

A static variable occupies a single fixed memory location

whereas

Instance variables of each object occupy separate memory locations.

The name of the class is StudentEnroll.

The instance variables are declared as id, name and branch.

The variable count is declared as static since it is common to the whole class.

Inside this project we will create the necessary classes to demonstrate the usage of Static variables.

Go back to slide 8 and show the code

Copy and paste the code

Now type the code which was shown in the earlier slide.

Delete the super keyword

Delete the super keyword from the generated code.

public StudentEnroll(String id, String name, String branch) {

this.id = id;

this.name = name;

this.branch = branch;

count++;

}

We also need to increment the value of the variable count by one every time an object is created.

So inside the constructor type,

count ++ semicolon.

{// Copy the code below

System.out.println(id+" " +name+" "+branch+" "+count);

}

So type public void showData( )

within brackets type the following code to print the values of id, name, branch and total number of students enrolled.

Inside this class we will have the main method.

So type main and then press ctrl+space to generate the main method.

StudentEnroll s1=new StudentEnroll("IT101","ADIL","IT");

Type s1.showData( );

Click on Run icon

Highlight the values of s1

Highlight 1

StudentEnroll s2=new StudentEnroll("EC101","AMAL","EC");

copy and paste, s2.showData();

Click on Run icon

Highlight values

Highlight 2

StudentEnroll s3=new StudentEnroll("CS101","CAROL","CS");

Type, s3.showData();

Click on Run icon

Hightlight the values

Highlight 3

We will create a few objects of StudentEnroll class to represent student enrollments.

So type the following code.

StudentEnroll s1 equals new StudentEnroll

Now we can pass the values of different arguments.

Within brackets type IT101 as id, ADIL as name and IT as branch.

Now let us invoke the showData method to print the enrollment details. So type

s1.showData( ).

Now run the Demo program.

We can see that the values of the instance variables corresponding to s1 get printed.

Also note that the value of the number of student enrollments is 1.

This is because we have created only 1 object.

Now type the following code to create one more object s2.

The showData method can be called once again using s2.

Again run the Demo Program.

We can see that the values of the instance variables corresponding to s2 get printed.

Also note that the value of number of student enrollments is updated to 2 for both s1 and s2.

Now create one more object s3.

Now Let us Invoke the showData method again using s3.

Run the Demo Program again.

We can see that the values of the instance variables corresponding to s3 get printed.

Also note that the value of number of student enrollments is now updated to 3 in all the cases.

Now we can understand that the value of the number of student enrollments is common to all the objects.

The static modifier, is also used along with the final modifier.

It is done to define a constant which is common to the entire class.

Conventionally, the names of such constant variables are spelled in uppercase letters.

As an Example, if all the students are getting enrolled to the same organisation say IITB.

It can be represented by using a constant static variable say ORG_NAME.

Usually we declare such constants

with a public visibility.

directly from another class by using its own

class name.

For Example:

StudentEnroll.ORG_NAME;

In StudentEnroll class and type,

public static final String ORG_NAME = “IITB”;

Inside the Demo class, type System.out.println(“ORGANISATION:”+StudentEnroll.ORG_NAME);

Highlight ORG_NAME and StudentEnroll

Run the Demo program

Point to ORGANISATION: IITB

Open the StudentEnroll class and type the variable declaration as

public static final String ORG_NAME = “IITB”;

Now go to the Demo class and type the following code

Here you can see that ORG_NAME is accessed by using its class name StudentEnroll.

Now run the Demo Program again.

We can see that the ORGANISATION name is getting printed as IITB.

Summary

In this tutorial we have learnt about

• What is a static variable and when it is used
• How to create and invoke the static variables

Assignment

Design a class CarService to represent a Car Service Station.

This class should contain variables to represent the following details.

Name of the Service Station.

Car make, model and reg. no - which are in for service.

No. of Cars in for Service.

Identify the instance variables and static variables.

Declare them using suitable keywords.

Define a constructor to initialize the values for Car make, model and regno.

Define a method show( ) to print the values of all the variables.

Also create a Demo class containing the main method to verify the results.

i.e

Create a few objects of CarService.

Invoke the show( ) method using these objects.

Also access the static variables directly using the class name.

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