Final Keyword in Java

Variables are useful when you need to store information that can be changed as the program runs. However, there may be certain situations in the program in which the variable’s value should not be allowed to modify. It accomplished using a special type of variable known as the final variable. The final variable also called a constant variable. It is a variable with a value that cannot be modified during the execution of the program. To declare a final variable, use the final keyword before the variable declaration and initialize it with a value. Its syntax is

final datatype varName = value;

For example:

final double PI = 3.141592;

This statement declares a variable PI of type double, which initialized to 3.141592, that cannot be modified. If an attempt made to modify a final variable after it initialized, the compiler issues an error message “cannot assign a value to final variable PI“.
You can also defer the final variable’s initialization, such a final variable known as a blank final. However, it must be initialized before it used.
Final variables are beneficial when one need to prevent accidental change to method parameters and with variables accessed by anonymous class.

Now consider a program to calculate the area of the circle,

//Area of the circle
public class AreaCircle {
 Public static void main(String[] args) {
   int r = 10; //initializing radius
   final double PI = 3.141592; //final variable
   double area;
   //PI = 3.142; will generate an error
   area = PI * r * r;
   System.out.println("Area of circle is -->"+area);
 }
}
Output: Area of circle is --> 314.1592

Final Class

Just like final methods, a class can also be declared final. A final class is a class that cannot be used as a superclass (i.e. a Class cannot extend final class). To declare the class as final, precede the class header with the final keyword. So the class header will look like.

final class ClassName { }

When you declare a class to be final, all methods of this class are implicitly final. Another class cannot extend the final class, so no class can be able to override any of the final class methods. For example, String and StringBuffer classes are examples of final classes.

public final class HourlyEmployee extends Employee {
  // class definition
}

Final Method

Sometimes you may want to prevent a subclass from overriding a method in your class. To do this, add the keyword final at the start of the method declaration in a superclass. Any attempt to override a final method will result in a compiler error.

//program to show final method
class Base {
  final void display {
    System.out.println("Base method called");
  }
}
class Derived extends Base {
  void display() //cannot override {
    System.out.println("Base method called");
  }
}
class FinalMethodDemo {
  public static void main (String [] args) {
   Derived d =new Derived();
   d.display();
  }
}

Explanation: On compiling the above program, it will display an error “display() in Derived cannot override display() in Base; the overridden method is final.

The following points should be remembered while using final methods.

• Private methods of the superclass are automatically considered to be final. It is because you cannot override a method in the subclass. Similarly, the static methods are also implicitly final as they cannot be overridden either.

• Since the compiler knows that a subclass cannot override final methods, these methods can sometimes provide performance enhancement by removing calls to final methods and replacing them with the expanded code of their declarations at each method call location. This technique of replacing code is known as inlining the code. There is no transfer of control between the calling and the called method that result in removing the method call overhead
which in turn improves the execution time.
Methods made inline should be small and contain only a few lines of code. If it grows in size, the execution time benefits become a very costly affair.
• A final’s method declaration can never change, so all subclasses use the same method implementation and call to one can be resolved at compile time. It is known as static binding.

Note: The getclass() and wait() methods of the Object class are declared final since their implementation is essential for an object’s proper behavior. So those methods cannot be overridden by any class.