How does the “final” keyword in Java work? (I can still modify an object.)

Question

In Java we use final keyword with variables to specify its values are not to be changed. But I see that you can change the value in the constructor / methods of

Answer 1

Following are different contexts where final is used.

Final variables A final variable can only be assigned once. If the variable is a reference, this means that the variable cannot be re-bound to reference another object.

class Main {
   public static void main(String args[]){
      final int i = 20;
      i = 30; //Compiler Error:cannot assign a value to final variable i twice
   }
}

final variable can be assigned value later (not compulsory to assigned a value when declared), but only once.

Final classes A final class cannot be extended (inherited)

final class Base { }
class Derived extends Base { } //Compiler Error:cannot inherit from final Base

public class Main {
   public static void main(String args[]) {
   }
}

Final methods A final method cannot be overridden by subclasses.

//Error in following program as we are trying to override a final method.
class Base {
  public final void show() {
       System.out.println("Base::show() called");
    }
}     
class Derived extends Base {
    public void show() {  //Compiler Error: show() in Derived cannot override
       System.out.println("Derived::show() called");
    }
}     
public class Main {
    public static void main(String[] args) {
        Base b = new Derived();;
        b.show();
    }
}