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

Learning Objectives

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

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

Software Requirements

• Ubuntu 12.04
• JDK 1.7
• Eclipse 4.3.1

• Ubuntu 12.04
• JDK 1.7 and
• Eclipse 4.3.1

Prerequisites

• Basic knowledge of Java and Eclipse IDE
• Knowledge of classes, objects and instance variables
• For relevant tutorials, please visit

http://www.spoken-tutorial.org

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?

• A static variable is a variable which is associated

with the entire class

• It is also called a class variable
• It is declared using the static keyword

It is also called a class variable.

It is declared using the static keyword.

In this tutorial we will see it in detail.

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

Type StudentEnroll and hit Enter

Go back to slide 8 and show the code

Copy and paste the code

Static Variable-Example

(Proceed with the Slide)

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

private String id, name, branch;

public static int count=0;

}

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.

When a class is loaded-

• A static variable in a class occupies

a single fixed memory location

• Instance variables of each object occupy

separate memory locations

Whereas Instance variables of each object occupy separate memory locations.

Select Generate Constructor using Fields

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, total number of students enrolled.

And then type name as Demo.

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( );

Highlight the values of s1

Highlight 1

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

copy and paste, s2.showData();

Click on Run icon

Highlight 2

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

Type, s3.showData();

Click on Run icon

Highlight 3

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

So type 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 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 value of number of student enrollments is common to all the objects.

Static Variables - As Constants

• 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

• The names of constant variables are

spelled in upper-case letters

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 spelt in uppercase letters.

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”;

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.

If the name is composed of more than one word, the words are separated by an underscore.

Usually we declare such constants with a public visibility.

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

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

Summary

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

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 initialise 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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