Introduction
Final keyword in java is basically used for imposing restriction to user.
final keyword is used in many context, which are mentioned below :
- Variable
- Method
- Class
It means, we can use final keyword in variables, methods and classes in Java.
Let’s learn each in details :
Java Final Variable
Final variables in Java are nothing but constants. Once the final variable is initialized, we cannot change its value.
Declaration : final <data_type> <variable_name> = <value>;
Example : Final Keyword in Java : Final Variable
public class Circle {
final int RADIUS=30; //declaring final variable
void draw(){
//trying to change final variable's value
RADIUS=400;
}
public static void main(String args[]){
Circle obj=new Circle();
obj.draw();
}
}Output:
Circle.java:7: error: cannot assign a value to final variable RADIUS RADIUS=400; ^ 1 error
As in above example, we have declared variable radius as final. So, we are not allowed to change its value. Hence, compile time error is occured.
As per Java naming convention, constants should be declared in CAPS letter.
Blank final variable
As we know, We can apply final keyword in Java with variables.
Blank Final Variables are those final variables which are not initialized at the time of declaration. These are also known as uninitialized final variable or blank final variable.
These type of variables can only be initialized in constructor.
Declaration : final <data_type> <variable_name> ;
Example : Final Keyword in Java : Blank Final Variable
public class Circle {
final int RADIUS; //declaring blank final variable
Circle(){
RADIUS=40;
}
void draw(){
System.out.println("Drawing Circle of Radius - "+RADIUS);
}
public static void main(String args[]){
Circle obj=new Circle();
obj.draw();
}
}Output:
Drawing Circle of Radius - 40
Now, Let’s see what happens if we try to initialize the blank final variable outside the constructor :
public class Circle {
final int RADIUS; //declaring blank final variable
void draw(){
RADIUS=40;
System.out.println("Drawing Circle of Radius - "+RADIUS);
}
public static void main(String args[]){
Circle obj=new Circle();
obj.draw();
}
}Output:
Circle.java:8: error: cannot assign a value to final variable RADIUS RADIUS=40; ^ 1 error
What is the use of Blank Final Variable in Java?
Let’s try to understand it with a real life example :
- Suppose, we have one
Employeeclass and it has a fieldEmployee_Id. Employee_Idshould befinalas it is a constant.- We cannot initialize the
Employee_Idat the time of declaration otherwise eachEmployeewill have sameEmployee_Id. - Thus, we make
Employee_Idas Java Blank Final Variable.
Let’s see the below example :
public class Employee {
final int EMPLOYEE_ID; //declaring blank final variable
Employee(int emp_id)
{
EMPLOYEE_ID=emp_id;
}
void print()
{
System.out.println(EMPLOYEE_ID);
}
public static void main(String args[]){
Employee employee1=new Employee(1002);
Employee employee2=new Employee(1003);
employee1.print();
employee2.print();
}
}Output:
1002 1003
Static Blank final variable
Java blank final variable can be static as well.
The Static Final variables can not be initialized in constructors. It can only be initialized in static block.
Declaration : static final <data_type> <variable_name>;
Example : Final Keyword in Java : Static Blank Final Variable
public class Employee {
static final int EMPLOYEE_ID; //declaring blank final variable
static
{
EMPLOYEE_ID=1234;
}
void print()
{
System.out.println(EMPLOYEE_ID);
}
public static void main(String args[]){
Employee employee1=new Employee();
employee1.print();
}
}Output:
1234
Java Final Method
Final keyword in Java can be used with methods as well. A Final method in Java cannot be overridden.
It means though sub class can call the final method of parent class but cannot override it.
Declaration : final <return_type> <method_name>(){}
Example : Final Keyword in Java : Final Method
class Shape {
final void draw()
{
System.out.println("Draw.....");
}
}
public class Circle extends Shape{
void draw()
{
System.out.println("Draw Circle.....");
}
public static void main(String args[]){
Circle obj=new Circle();
obj.draw();
}
}Output:
Circle.java:12: error: draw() in Circle cannot override draw() in Shape void draw() ^ overridden method is final 1 error
Hence, we cannot override the Java final method.
Now, Let’s look at another example to see if we can call the final method of parent class :
class Shape {
final void draw()
{
System.out.println("Draw.....");
}
}
public class Circle extends Shape{
public static void main(String args[]){
Circle obj=new Circle();
obj.draw();
}
}Output:
Draw.....
Java Final Class
We cannot inherit the Java Final Class. It means, java class cannot extend any final class.
Declaration : final class <class_name>{}
Example : Final Keyword in Java : Final Class
final class Shape {
void draw()
{
System.out.println("Draw.....");
}
}
public class Circle extends Shape{
public static void main(String args[]){
Circle obj=new Circle();
obj.draw();
}
}Output:
Circle.java:10: error: cannot inherit from final Shape
public class Circle extends Shape{
^ 1 error
Additional Points
- Variables declared in an interface in Java are by default final.
- Method’s parameters can be declared as final.
- final , finally and finalize are three different terms.
- Final variables should be declared in Capital letters as per convention. It is not mandatory though.
- We can not declare a constructor as final.
Frequently Asked Questions
Yes, It is inherited. We can use it but cannot override it.
Yes, but in constructor.
Any parameter can be declared as final. Its value cannot be changed.
No, It’s not allowed.
Further Readings:
Conclusion
Final keyword in Java is used in three contexts mainly : variable, method and class. It basically imposes restrictions such as no change allowed, no overriding, no inheritance etc.
This is one of the most asked Java interview question.
Thanks for Reading!
