Difference between revisions of "Java/C3/Using-final-keyword/English"

We will also learn about -

• final variables,
• final methods,
• final classes

Software Requirements

• Ubuntu Linux version 12.04
• JDK 1.7
• Eclipse 4.3.1

Prerequisites

• Basic knowledge of Java and Eclipse IDE.
• and knowledge of Subclassing and Method overriding.

If not, for relevant Java tutorials, please visit our website.

final keyword

final is a keyword or reserved word in Java.

It can be applied to variables, methods and classes.

Slide 6

final variable is a variable whose value cannot be changed.

That is, it will be a constant.

I had already created a project named MyProject in the previous tutorial.

We have made the variable name as final.

cannot assign a value to final variable name

name=newName;

The final field Employee.name cannot be assigned

We can initialize a final variable only once.

So we will comment the method setName which modifies the variable name.

Save the class

Type // before manager.setName("Nikkita Dinesh");

Come to the main method and comment the line manager.setName("Nikkita Dinesh");

We commented this line as it was instance of method setName.

We have already commented setName method in Employee class.

Name: Sneha

Email: abc@gmail.com

Manager of: Accounts

We got the output,

Name: Sneha

Email: abc@gmail.com

Manager of: Accounts

Now come to the final variable name. in Employee class.

That is remove “sneha”.

Uncomment the method.

cannot assign a value to final variable name

name=newName;

The final field Employee.name cannot be assigned

That is it cannot be modified anywhere else in the program.

We have already learnt what a constructor is earlier.

Employee() { name="Sneha";

}

Employee, parentheses, open and close curly brackets

and inside the curly brackets, lets initialize the variable name with value Sneha.

Enclose the method inside /* */

Name: Sneha

Email: abc@gmail.com

Manager of: Accounts

We have made the final variable as static.

cannot assign a value to final variable name

name="Sneha";

The final field Employee.name cannot be assigned

Further slide 9.1 and 9.2 [i.e. 25,26] contains one extra word block in second bullet,doneSlide 7

They must be assigned a value with their declaration.

Or they must be declared in a static block.

Creating a new object would change the static variable.

This is not allowed if the static variable is final

Delete Employee and type static.

For that, in the Employee class, go to the constructor. we created

Here instead of Employee(), we will type static.

We have created a static block.

Name: Sneha

Email: abc@gmail.com

Manager of: Accounts

We have successfully initialized static final variable.

We have made the parameter as final.

Name: Sneha

Email: abc@gmail.com

Manager of: Accounts

Inside the method we will type,

newEmail is equal to inside double quotes abc@gmail.com semicolon.

We have modified the final variable newEmail.

final parameter newEmail may not be assigned

The final local variable newEmail cannot be assigned.

So lets remove the variable modification. <<PAUSE>>

Come to the method getDetails in employee class.

Add final keyword before method getDetails.

Cannot override the final method from Employee

class Manager overrides final method getDetails.()

Come to the method getDetails() in Manager class. This is because if you make any method as final, you cannot override it. Manager class method getDetails overrides getDetails method in Employee class.

private methods are not inherited by the child class.

So we can add a method getDetails() in the child class.

You can try this as an assignment.

In Employee class remove final keyword before method getDetails. Remove static keyword before final variable name.

Employee() { name="Sneha";

}

Now we will learn whether constructor can me declared as final or not

For that we will again create a constructor.

So instead of static we will type, Employee parentheses.

modifier final not allowed here

Illegal modifier for the constructor in type Employee

we will remove the final keyword before the constructor.

The type Manager cannot subclass the final class Employee

The method setEmail is undefined for the type Manager

The type Manager cannot subclass the final class Employee

To know the actual error, lets come to TestEmployee class and comment the lines manager.setName("Nikita Dinesh"); manager.setEmail("abc@gmail.com"); manager.setDepartment("Accounts");

Save the class and run the program The actual error is the type manager cannot subclass the final class Employee. Here Manager class extends Employee class. So lets come back to Employee class and remove the final keyword. Come to the TestEmployee class Uncomment the lines manager.setEmail("abc@gmail.com"); manager.setDepartment("Accounts");

Save the class and run the program. We got the desired output.

Summary

• When to invoke final keyword.
• What are:
  • final variables,
  • final methods and
  • final classes

Assignment

• • Are you sure about the name of the tutorial? Pls chkThe bike vehicle was given as an assignment in the previous tutorial.
    We need to frame it properly.Pls frame it properly.doneRepeat the steps of Using final keyword tutorial for the Bike and Vehicle class.
  • Write down the classes in java that are final classes.

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This is Trupti Kini from IIT Bombay signing off.

Thanks for joining.