Array assignment doubt??

[Joe San]

Hi Guys,

Consider the code below from the K&B book,

class Dims{
 public static void main(String args[]){
  int [][] a = {{1,2}, {3,4}};
  int [] b = (int[]) a[1];
  Object o1 = a;
  int [][] a2 = (int [][])o1;
  int [] b2 = (int[])o1;
  System.out.println(b[1]);
 }
}

Please help me understand the O/P as to why it gives a run time error??

[Ernest Friedman-Hill]

Well, these two lines:

int [][] a2 = (int [][])o1;
int [] b2 = (int[])o1;

try to cast o1 to two different, incompatible types. Only one of them can possibly succeed; these two types can't be interconverted. The first line succeeds, since o1 points to the original "a", which is indeed an int[][]. Therefore the second line fails, because o1 is not an int[].

[Peter Heide]

Ernest answer is correct.

o1 is an Object that points to a two-dimensional array.
If you want you can cast it to a two-dimensional array.
Then you can assign one dimension to the one-dimensional array b2.

The following code works:

 
class test
{ 
public static void main(String args[]) {  
int [][] a = {{1,2}, {3,4}};  
int [] b = (int[]) a[1];  
Object o1 = a;  
int [][] a2 = (int [][])o1;  
int [] b2 = ((int [][])o1)[1];  
System.out.println(b[1]); // Output: 4
System.out.println(b2[1]);  // Output: 4
}
}
 

[Joe San]

So, the point is when we cast a one dimensional array to a 2 dimensional array and vice versa, it gives us a run time error?? Am I right guys??

Please confirm.

[Prabhu Venkatachalam]

yes, when you try to cast object to incompatible types result in java.lang.ClassCastException exception which is runtime exception.

hth,

[Peter Heide]

Dear Jothi Shankar Kumar, I think you are right.