Array- can't understand the code.

[Nabila Mohammad]

1  class Dims {
2public static void main(String[] args) {
 
3       int[][] a={ {1,2}, {3,4} };
4int[] b= (int[]) a[1];
5Object o1=a;
6int[][] a2=(int[][])o1;
7int[] b2=(int[]) o1;
8System.out.println(b[1]);
}

     }
  

Can some one please explain what's going on this code.
What does line 4 mean.
In line 5 why is a 2-dimensional array being assigned to a variable of type Object.
What is line 6&7 . Some kind of casting?

I got a runtime error(Class Cast exception) because of line 7 which if removed will give the output 4.

[Question from K&B]

[Gaurav Joshi]

Code Explained with comment

1 class Dims {
2public static void main(String[] args) {

3 int[][] a={ {1,2}, {3,4} }; // Creating two dimensional Array with
//a[0] = {1,2} and a[1] = {3,4}
4int[] b= (int[]) a[1]; // assigning a[1] to b
5Object o1=a; // Remember a is also an object so
// upcasting to Object
6int[][] a2=(int[][])o1; // assigning a to a2 with downcasting
7int[] b2=(int[]) o1; // Run Time exception as ol can only be
//type casted to 2- d array
8System.out.println(b[1]);
}

}

Thanks and Regards,
Gaurav Joshi

[Nadeem Khan]

int[][] a={ {1,2}, {3,4} };
//Declares an array object referred by a. a's array object contains references to two array objects:
a[0] --refers to--> {1,2}
a[1] --refers to--> {3,4}
Therefore we actually are creating 3 array objects on the heap here. One referred by a, second by a[0] and third by a[1].



4int[] b= (int[]) a[1];
// b refers to the 1D array object referred by a[1], so casting allowed & not necessary here.

5Object o1=a;
//Since any array object is also an Object, we can refer to a by an Object type variable o1.

6int[][] a2=(int[][])o1;
//Since o1 actually refers a 2D array type, we CAN cast it back to a 2D array type.

7int[] b2=(int[]) o1;
//Since o1 actually refers a 2D array type, we CANT cast it back to a 1D array type.

8System.out.println(b[1]);
//b --> a[1]. Hence b[1] = a[1][1] = 4!




Can some one please explain what's going on this code.
What does line 4 mean.
In line 5 why is a 2-dimensional array being assigned to a variable of type Object.
What is line 6&7 . Some kind of casting?

I got a runtime error(Class Cast exception) because of line 7 which if removed will give the output 4.

--#@ I hope that cleared a bit @#--

[yu yong]

Gaurav Joshi And Nadeem Khan`s reply are all right.
I think the problem is clear.

[Nabila Mohammad]

Thanks alot guys..That cleared alot!
Just one question..

Object o1=a;

Can a two dimensional array be assigned to o1 of type Object (which is not 2-dimensional) and simply an object.

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

Why do we need to cast o1(ie. a as Object o1=a) when it is already 2-dimensional.
[I am getting an error of incompatible type when I am trying to compile it with out casting.]


Thanks.

[Henry Wong]

Can a two dimensional array be assigned to o1 of type Object (which is not 2-dimensional) and simply an object.

All arrays are objects. So, a two dimensional array, which is an array of array, is an object.

Why do we need to cast o1(ie. a as Object o1=a) when it is already 2-dimensional.
[I am getting an error of incompatible type when I am trying to compile it with out casting.]

All arrays are objects, but not all objects are arrays. So the Java compiler doesn't know if an object can be casted to an array, implicitedly.

Henry

[Nabila Mohammad]

Thanks.
That made it clear.