Java OCA OCP Practice Question 1838

Question

Which is the first line that will cause compilation to fail in the following program? 

class MyClass extends Main  {  }
// Filename: Main .java
class Main {/*from w w w  .  ja v a 2 s .  c  om*/
   public static void main (String args []){
      Main a = new Main ();
      MyClass b = new MyClass ();
      a = b;  // 1
      b = a;  // 2
      a =  (MyClass) b; // 3
      b =  (MyClass) a; // 4
    }
}

Select 1 option

  • A. At Line 1.
  • B. At Line 2.
  • C. At Line 3.
  • D. At Line 4.
  • E. None of the above.


Correct Option is  : B

Note

Casting a base class to a subclass as in : b = (MyClass) a; is also called as narrowing and needs explicit cast.

Casting a sub class to a base class as in: Main a = b; is also called as widening and does not need any casting.

For example, consider two classes: Printable and Car, where Car extends Printable Now, Printable a = new Car (); is valid because a car is definitely an Printable.

So it does not need an explicit cast.

But, Car c = a; is not valid because 'a' is an Printable and it may be a Car, a Truck, or a MotorCycle, so the programmer has to explicitly let the compiler know that at runtime 'a' will point to an object of class Car.

Therefore, the programmer must use an explicit cast: 

Car c =  (Car) a;



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