Demystifying 'final' Keyword in Java | Interview Questions | Interview Q Point

 Demystifying Final Keyword in Java

Final a keyword in Java. Final is non-access modifier mainly used in 3 different context:

1. Variable

2. Method

3. Class

i.) Final Variable:  When a variable is marked as 'final', then we cannot change it's value.

Example: 

// a final static variable PI
static final double PI = 3.141592653589793;

ii.) Final Method: When a method is marked as 'final', then it cannot be overridden by inheriting class. 

class A 
{
    final void m1() {
        System.out.println("This is a final method.");
    }
}
class B extends A 
{
    void m1() {
        // COMPILE-ERROR! Can't override.
        System.out.println("Illegal!");
    }
}

iii.)  Final Class: When a class is marked as 'final' then other class cannot extend it. 

Example: 

final class A {
     // methods and fields
}
// The following class is illegal.
class B extends A  { 
    // COMPILE-ERROR! Can't subclass A
}

Ques: Can we use 'final' with Constructor? 

Ans: No, Since a final method cannot be overridden by any subclasses. And the final modifier prevents a method from being modified in a subclass.

So, in case we try to mark constructor as 'final', the Constructor cannot be inherited if a constructor is being called from child class. Therefor java does not allow final keyword before a constructor. if we'll try to add final then we'll get compile time error.

Example:

public class Example {
   public static void main(String args[]){
      int num;
      final public Example(){
         num = 30;
      }
   }
}

Output:

Exception in thread "main" java.lang.Error: Unresolved compilation problems:
   Syntax error, insert "enum Identifier" to complete EnumHeaderName
   Syntax error, insert "EnumBody" to complete BlockStatement
   Syntax error, insert ";" to complete Statement
   at newJavaExamples.Example.main(Example.java:6)