Array Question

[Harry Singh]

Hi,
Just got across this question in one of the mock exams.. and not surprisingly i gave the answer wrong...

1.  class Amber {
2.    public static void main(String[] args) {
3.      int[][] a = {{1,2},{0,1,2},{-1,0,2}};
4.      Object[] obj = (Object[])a.clone();
5.      for(int i = 0;i<obj.length; i++) {
6.        int[] ia = (int[])obj[i];
7.        System.out.print(ia[i]);
8.      }
9.    }
10. }

What is the result of attempting to compile and run the above program?
a. Compiler error at line 3.
b. Compiler error at line 4.
c. Compiler error at line 5.
d. Compiler error at line 6.
e. Compiler error at line 7.
f. Run time error.
g. None of the above.
I gave the answer as B, but the correct answer is G. theres absolutely no problems with the code.. According to me, u can assign a Two dimensional array to one D array.. so i thought line 4 will give a compilation error.. So where am i wrong in this ?
Regards,
Harry

[Jose Botella]

I think that accordirng to JLS 5.5 (Object[]) a.clone(); should produce a ClassCastException.
Let's see how the 1.4.2_01 compiler is fooling us:

class Test {
    public static void main(String[] args) {
        Object o = (int[]) new int[] {} ;
        o = (Test[]) new Test[] {};
        o = (Integer[]) new Integer[] {};
        o = (Object[]) new Object[] {};
        o = (String[]) new String[] {};
    }
}
/*
public static void main(java.lang.String[]);
  Code:
   0:   iconst_0
   1:   newarray int
   3:   checkcast       #2; //class "[I"
   6:   astore_1
   7:   iconst_0
   8:   anewarray       #3; //class Test
   11:  checkcast       #4; //class "[LTest;"
   14:  astore_1
   15:  iconst_0
   16:  anewarray       #5; //class Integer
   19:  checkcast       #6; //class Integer;"
   22:  astore_1
   23:  iconst_0
   24:  anewarray       #7; //class Object
   27:  checkcast       #8; //class Object;"
   30:  astore_1
   31:  iconst_0
   32:  anewarray       #9; //class String
   35:  checkcast       #10; //class String;"
   38:  astore_1
   39:  return
}
*/

I write (int[]) new int[] {} and the compiler cast the object to int[]
However I write (Integer[]) new Integer[] {}; and the compiler cast the object to Integer

[Steve Lovelace]

Sure can't state this with authority, my sense of what's happening is that an int[][] is an array of int arrays, i.e., it's an array of Objects, so everything in the example works.

[Lei Wu]

It sure won't cause any compile-time error in line 4. You know, a.clone() returns an object, while an object can be cast to anything at least during compile time. No exception will occur at run time, too . As Steve said, a.clone is actually an array of int[], it can be assigned to an array of object. In fact, you can even write line 4 like this:
Object[] obj = (int[][]) a.clone();
It still works correctly.
Hope this help.

[Harry Singh]

so what should be the output of this array and why.. which all int values will be there in a single D array. Will al the int values be there?

[Maulin Vasavada]

Hi Harjindar
So, if I try to explain this output,
lets consider int[][]a = new int[][]{{1,2},{0,1,2},{-1,0,2}};
as
int[] a1 = new int[]{1,2};
int[] a2 = new int[]{0,1,2};
int[] a3 = new int[]{-1,0,2};
int[][] a = new int[][]{ a1, a2, a3};
Now, when we casted a.clone() to Object[] it would become array of 3 objects which are a1,a2 a3 which are again arrays.
So in the for loop we have, obj.length would be = 3. It means it will go from 0..2. Now, here is what we have,
obj[0] = a1;
obj[1] = a2;
obj[2] = a3;
So far so good?
Now, in the loop we are casting each obj[i] in int[] and then printing i th element (the loop index) so first time it would print,
obj[0][0],
second time,
obj[1][1]
third time,
obj[2][2]
right?
you get my point? So the output would be 1,1,2.
Regards
Maulin

[Jose Botella]

Welcome to the Ranch Lei.
__________________________________________________________________________
I understand that a multimensional array can be thought as an array of objects. However the runtime type of the object returned by clone is still int[][].
The cast "(Object[]) new int[][] {{}};" conflicts --to my knowledge-- with JLS 5.5
Isn't suspicious that the compiler checks this cast (int[][]) new int[][] {{}} against the type int[][], but in this one (Object[]) new int[][] {{}} it cheks against Object ?

class Test {    
    public static void main(String[] args) {
       Object o = (Object[]) new int[][] {{}};
       o = (int[][]) new int[][] {{}}; 
    }    
}
/*
public static void main(java.lang.String[]);
  Code:
   0:   iconst_1
   1:   anewarray       #2; //class "[I"
   4:   dup
   5:   iconst_0
   6:   iconst_0
   7:   newarray int
   9:   aastore
   [B]10:  checkcast       #3; //class Object;"[/B]
   13:  astore_1
   14:  iconst_1
   15:  anewarray       #2; //class "[I"
   18:  dup
   19:  iconst_0
   20:  iconst_0
   21:  newarray int
   23:  aastore
   [B]24:  checkcast       #4; //class "[[I"[/B]
   27:  astore_1
   28:  return
*/

[Lei Wu]

To Jose Botella:

The cast "(Object[]) new int[][] {{}};" conflicts --to my knowledge-- with JLS 5.5

No, it doesn't conflicts with JLS 5.5.
Now, let's first look at what JLS 5.5 says:
<1> If S is an array type SC[], that is, an array of components of type SC :
<1.1> If T is an array type TC[], that is, an array of components of type TC, then a compile-time error occurs unless one of the following is true:
<1.1.1> TC and SC are reference types and type SC can be cast to TC by a recursive application of these compile-time rules for casting.
So, here SC[] is int[][], which is an array of int[], SC is hence int[]. TC[] is Object[], and TC is of type Object. Since int[] can be cast to Object at compile-time, it is correct to cast int[][] to Object[], according to the above rule. Actually, it's a narrowing reference conversion from int[][] to Object[].

[Jose Botella]

You are right Lei.
I just though SC was int[][] but it is int[]
In fact no checking is necessary at runtime according to

The cast can be determined to be correct at compile time. A cast from the compile-time type S to compile-time type T is correct at compile time if and only if S can be converted to T by assignment conversion (�5.2)

Thank you for adjusting my knowledge!
___________________________________________________________________________
Object[] o = new int[][] {{}};
it is a widdening reference conversion, you do not need a cast for the assignment.
[ September 24, 2003: Message edited by: Jose Botella ]