object refrence variables

[Puspender Tanwar]

Given the following program, which statements are true?

public class TestClass{ 
   public static void main(String args[]){ 
      A[] a, a1; 
      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 { } 

A. Compile time error at line 3.
B. The program will throw a j ava.lang.ClassCastException at the line labelled 2 when
run.
C. The program will throw a j ava.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
but according to me if C is correct then why not B. As a1 and a both refer to the same array object.

[Paul Clapham]

Puspender Tanwar wrote:As a1 and a both refer to the same array object.

But they don't. Not at the line with the comment //2 at the end, anyway.

[Puspender Tanwar]

Paul Clapham wrote:But they don't. Not at the line with the comment //2 at the end, anyway.

correct me if I am wrong
in starting both a and a1 were pointing to the same new A[10] but after //1 a and b will refer to new B[20] and a1 will refer to new A[10].
so no error or exception is at //2

[Paul Clapham]

Yes, that looks right to me. (And it makes the answers in your original post come out correct, too.)

By the way could you please tell us where that code came from? Read our FAQ entry QuoteYourSources (<-- click that link) for information about why we want to know that.

[Puspender Tanwar]

Paul Clapham wrote:By the way could you please tell us where that code came from? Read our FAQ entry QuoteYourSources (<-- click that link) for information about why we want to know that.

I have a book -> webpage

test1- question number 52

[R K Singh]

Let me try to explain :

a= new A[10]; a1= a;
b= new B[20];
a= b; // 1
b= (B[]) a; // 2
b= (B[]) a1; // 3


Following are references to memory location :

a = 1000 -> A[10]
a1=1000 -> because of a1 = a
b = 2000 -> B[20]

because of line //1
a = 2000

NOTE, a1 is still pointing to 1000

//2 require casting,

//3 will throw run time casting exception because a1 points to A[]

HTH