Array Conversion

[Micheal John]

From web page

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

In the line 2,it is said that the two dimensional array is converted to one dimensional array. If two dimensional array is converetd to one dimensiona array, how the values in 2-d array is given to 1-d array? in the above pgm the o/p is 112..how?

[Mark Uppeteer]

Heh, tricky one.

The 2 dimensional array is converted to an array of objects (3 objects).
Those objects are arrays again but that is not known for obj.

Later on , the 3 objects in the 1 dimensional array are casted again to int[] which is perfectly possible because an array is an object. Then the loop prints the values...

regards,
Mark

[Burkhard Hassel]

Hi ranchers,

int[][] a = { { 1, 2 }, { 0, 1, 2 }, { -1, 0, 2 } }; 
Object[] obj = (Object[]) a.clone();

the last line makes an Object array with three references pointing to the ^ in

int[][] a = { { 1, 2 }, { 0, 1, 2 }, { -1, 0, 2 } }; 
              ^         ^            ^

perhaps change the output line from
// System.out.print(ia[i]);
to this three lines:
System.out.println(java.util.Arrays.toString(ia));
boolean sameRef = ( obj[i] == a [i] ) ;
System.out.println(sameRef);


Then it prints:
[1, 2]
true
[0, 1, 2]
true
[-1, 0, 2]
true

The elements printed originally are in bold.

OK?



Yours,
Bu.

[Arpit Khandelwal]

Sounds simple.

Hmmm , the object array after "Object[] obj = (Object[])a.clone();" would look like : obj[0] = {1,2}
obj[1] = {0,1,2}
obj[2] = {-1,0,2}
now you are typcasting this object array in the loop "int[] ia = (int[])obj[i];"

so for i = 0 we will have int ia[] = {1,2} and when you say ia[0] output is =1.
similarly for i=1 we have int ia[] = {0,1,2} and ia[1] = 1 and you can draw similar conclusion for i=2.

I hope this answers your questions.

[Bob CHOI]

How to understand a (N) Demensional any type of array is subclass of (N-m)Demensional Object array. Code below verify the statement could be true!???

  
class A {
public static void main(Strings args) {
System.out.println(new int[10][10][10][10] instanceof Object[][]); //4D int array is subclass of 2D Object array
}
}

it prints out:

true

[ December 13, 2006: Message edited by: Bob CHOI ]
[ December 13, 2006: Message edited by: Bob CHOI ]

[Henry Wong]

Originally posted by Bob CHOI:
How to understand a (N) Demensional any type of array is subclass of (N-m)Demensional Object array. Code below verify the statement could be true!???

  
class A {
  public static void main(Strings args) {
    //4D int array is subclass of 2D Object array
    System.out.println(
        new int[10][10][10][10] instanceof Object[][]);
  }
}

it prints out:

true

You just need to remember two facts. In Java, (1) an array is an object, and (2) Java doesn't directly support N-dimensional arrays, it supports arrays of arrays. So...

So a 4 dimensional int array is really an array of an array of an array of an array of ints. And since an array of ints is an object, the 4D array of ints is also an array of an array of an array of objects. And since an array of objects is an object, the original 4D array of ints is also an array of an array of objects.

Henry

[Sanjeev Singh]

Henry,
This is really a very good explantion,hope one will not mess up the things when faces the questions on arrays of arrays... of arrays... of arrays..

[Bob CHOI]

Thanks, Henry.


Striving for digesting "array of array...."

[Bob CHOI]

You just need to remember two facts. In Java, (1) an array is an object, and (2) Java doesn't directly support N-dimensional arrays, it supports arrays of arrays. So...

So a 4 dimensional int array is really an array of an array of an array of an array of ints. And since an array of ints is an object, the 4D array of ints is also an array of an array of an array of objects. And since an array of objects is an object, the original 4D array of ints is also an array of an array of objects.

Henry

- an array is an object, in pseudo-Java

class X {}

Object o = new X[m]...[n]; //right is subtype of left

- an array can hold subtype of objects, in pseudo-Java

Object o[] = new X[m]...[n][n+1];

- "a (N) Demensional any type of array is subclass of (N-m)Demensional Object array" statement PROVED!