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Welcome to the Spoken Tutorial on Instance fields in Java.

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In this tutorial, we will learn:

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* About instance fields

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* To access the instance fields of a class

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* Modifiers for instance fields

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* And, why instance fields are called so?

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Here we are using:

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* Ubuntu version 11.10

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* jdk 1.6

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* And  Eclipse IDE 3.7.0

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To follow this tutorial, you must know

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how to create a class in Java using Eclipse.

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You must also know how to create an object for the class.

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If not, for relevant tutorials, please visit our website which is as shown.
http://www.spoken-tutorial.org

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We know that objects store their individual states in fields.

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These fields are declared without the  static  keyword.

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We will learn about static fields in the coming tutorials.

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Non-static fields are also known as instance variables or instance fields.

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Let us go back to the Student class we had already created.

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We can see that here roll_no and name are the instance fields of this class.

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Now, we will learn how to access these fields.

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For that, let us open the TestStudent class which we had already created.

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We can remove the statement for creating the second object.

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We will also remove the println statements.

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Now we will access the fields, roll_no and name,  of the student class using stud1 and the dot operator.

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So, for that,  type: System dot out dot println within brackets and double quotes The roll number is then plus stud1 dot  from the option provided select ' roll_no' press Enter  then semicolon.

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Next line, type: System dot out dot println within brackets and double quotes The name is plus stud1 dot select 'name' press Enter then semicolon.

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Now, save and run the file TestStudent.java. So press   Ctrl, S and Ctrl, F11.

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We get the output as:

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The roll number is 0.

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The name is null.

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This is because, we have not initialized the variables to any value.

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In Java, the fields cannot have random values.

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After the memory is allocated for the object, the fields are initialized to null or zero.

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This work is done by the constructor.

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We will learn about constructor in the coming tutorials.

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Now, we will initialize the fields explicitly and see the output.

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So, type: int   roll_no equal to 50  next line String name equal to within double quotes Raju.

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Now, save and run the file. Press Ctrl, S and Ctrl, F11.

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We get the output as expected; The roll number is 50.

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The name is Raju.

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This is because we have explicitly initialized the variables  in the Student class.

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We can see that here the fields have no modifier or  default modifier.

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Recall modifiers, we had discussed in Creating Classes.

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We can access the fields because both Student.java and TestStudent.java are in the same package.

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We can see that, here, they are in the same default package.

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We will learn about packages in the later tutorials.

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We will now change the modifier to private.

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So, before the field declarations, type: private. So, type: private space int roll_no=50;.

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Next line, private space String name = Raju;.

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Now save the file Student.java.

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We can see that we get errors in TestStudent.java.

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Hover the mouse over the error symbol.

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It says The field Student  dot roll number is not visible.

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And The field Student  dot name is not visible.

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This is because private fields can be accessed only within its own class.

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You can try accessing roll_no and name from the Student class itself.

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You will find that you can access them without any error.

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Now let us change the modifier to protected.

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Now  Save the file and Run the program.

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We see the output on the console. The Roll no is 50 The name is Raju.

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This is because protected fields can be accessed within the same package.

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Now let us see why  instance fields are called so?

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Instance fields are called so because their values are unique to each instance of a class.

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In other words each object of a class will have unique values.

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Let us go back to the TestStudent class.

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Here, we will create one more object of the Student class.

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So, type: next line, Student space stud2 equal to new space Student opening and closing brackets semicolon.

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We will now initialize both the objects in the TestStudent class.

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So, next line, type: stud1 dot select roll_no  press  Enter equal to 20  semicolon.

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Next line, type: stud1 dot select  name press Enter equal to within double quotes Ramu  semicolon press Enter.

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Thus we have initialized the fields for the first object.

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Now, we will initialize the fields for the second object.

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So, type: stud2 dot select roll_no equal to 30 semicolon.

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Next line, stud2 dot select name equal to within double quotes Shyamu  semicolon press Enter.

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Now, after the println statements, type: System dot out dot println within brackets and double quotes The roll number is plus stud2  dot select roll_no and  semicolon.

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System dot out dot println within brackets and double quotes The name is, plus stud2  dot select name and semicolon.

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Now, save and run the file. Press  Ctrl, s and Ctrl, F11.

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We get the output as follows.  The roll_no is 20, The name is Ramu The roll_no is 30, The name is shyamu.

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Here, both stud1 and stud2 are referring to two different objects.

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This means that the two objects have unique values.

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We can see that here.

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The first object has the values 20 and Ramu.

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The second object has the values 30 and Shyamu.

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Now, let us create one more object.

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So type Student space stud3 equal to new space Student  within brackets opening and closing brackets semicolon.

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We will now print the values  of the third object.

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So, type: System dot out dot println within brackets and double quotes The  roll_no is plus stud3  dot select roll_no semicolon.

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next line, type:  System dot out dot println within brackets and double quotes The  name is plus stud3  dot name semicolon.

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Now, save and run the file. So press Ctrl, S and Ctrl, F11 .

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We  can see that the third object contains the values 50 and Raju.

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This is because we had explicitly initialized the fields of the Student class to 50 and  Raju.

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Now, try de-initializing the fields and see the output for the third object.

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So in this tutorial, we learnt:

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* About instance fields.

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* Accessing the fields using dot operator.

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For self assessment,

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Create an  object emp2 in the TestEmployee class already created.

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Then initialize the values of the two objects using dot operator.

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Use 55 and Priya as values for first object.

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Use 45 and Sandeep as values for second object.

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Display the values for both the objects in the output.

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To know more about the Spoken Tutorial Project,

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watch the video available at [1].

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It summarizes the Spoken Tutorial project.

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If you do not have good bandwidth, you can download and watch it.

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The Spoken Tutorial project team:

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Conducts workshops using spoken tutorials.

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Gives certificates to those who pass an online test.

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For more details, please write to contact@spoken-tutorial.org

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Spoken Tutorial Project is a part of the Talk to a Teacher project.

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It is supported by the National Mission on Education through ICT, MHRD, Government of India.

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More information on this mission is available at [2].

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Thus we have come to the end of this tutorial.

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This is Arya Ratish from IIT Bombay, signing off. Thanks for joining.