Final Keyword Demo in Java.

FinalDemo.java

/*
 * Final Keyword Demo.
 *
 * This demo will generate 3 errors:
 *  - error: cannot inherit from final A
 *  - error: display() in B cannot override display() in A
 *  - error: cannot assign a value to final variable x
 *
 * Uses of final keyword can be used:
 *  - to prevent inheritance;
 *  - to prevent method overriding;
 *  - to declare the constant variables;
 *
 * How to run:
 *   javac FinalDemo.java
 *   java B
 */

final class A {
  final void display() {

  }
}

class B extends A { // Error: A final class cannot be extended.
  void display() { // Error: Final method cannot be overridden.
    final int x = 1000;
    x = 2000; // Error: A final variable cannot be modified.
  }
}

class C extends B {
  int x = 3000;
}