Java OCA OCP Practice Question 470

Question

Given the following program, which statements are true? 

// Filename: Main.java 
public class Main{ 
   public static void main (String args []){ 
      A [] a, a1; //from  w w  w.  j  av  a  2s.co m
      B [] b; 
      a = new A [10]; a1  = a; 
      b =  new B [20]; 
      a = b;  // 1 
      b =  (B []) a;  // 2 
      b =  (B []) a1; // 3 
    } 
} 
class A  {  } 
class B extends A  {  }

Select 2 options

  • A. Compile time error at line 3.
  • B. The program will throw a java.lang.ClassCastException at the line labelled 2 when run.
  • C. The program will throw a java.lang.ClassCastException at the line labelled 3 when run.
  • D. The program will compile and run if the (B[] ) cast in the line 2 and the whole line 3 is removed. E. The cast at line 2 is needed. 


Correct Options are  : C E

Note

The line 1 will be allowed during compilation, since assignment is done from a subclass reference to a superclass reference.

The cast in line 2 is needed because a superclass reference is assigned to a subclass reference variable. And this works at runtime because the object referenced to by a is actually of an array of B.

The cast at line 3 tells the compiler not to worry, that I know what I am doing and the object referenced by the super class reference (a 1) will actually be of class B at run time.

So there is no compile time error.

But at run time, this fails because the actual object is not an array of B but is an array of A.




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