nullPointerException

[basha khan]

plz tell me how nullpointerException is occuring?.
how it related to arrays?
int [][]arr={ {1,2},null}
arr[1][0]=10;//nullpointer or arrayindexException??
what will arr[1].length will return ?&why?

[Rajinder Yadav]

If you look at the initialization, you haven't allocated anything for row 1, so arr[1][0] doesn't point to a valid reference.
Under this condition the statement a[1].length doesn't make any sense either and will cause an exception to be thrown (not sure if it's a null pointer exception or array out of bound exception, anyone?)
[ January 24, 2002: Message edited by: Rajinder Yadav ]

[Valentin Crettaz]

basha,
We'd like to read the Javaranch Naming Policy and register again.
Thank you for your cooperation.

[flynn xi]

I have tried your code. These two places both throw NullPointerException.

[mark stone]

true in the declaration second row is null.
but say we now allocate like
a[1][0]=10; and a[1]a[1]=15;
so now the second row is there and members are allocated. but still it gives NullPointerException.
why ? or is it that the declaration is the final word and it shall always remain null ? can someone let us know....

Originally posted by Rajinder Yadav:
If you look at the initialization, you haven't allocated anything for row 1, so arr[1][0] doesn't point to a valid reference.
Under this condition the statement a[1].length doesn't make any sense either and will cause an exception to be thrown (not sure if it's a null pointer exception or array out of bound exception, anyone?)
[ January 24, 2002: Message edited by: Rajinder Yadav ]

[R K Singh]

HI
I think
arr[1][0] is equal to null, i.e
arr[1][0] = null;
now array at arr[1][0] does not point to any Array object ... it is null.
hence when we try to allocate anything it throws NPE.
To assign any thing first you have to do :
arr[1] = new int[3];
now you can get arr[1].length
as now it contains refernce to an object and which has a legth of 3
CMIW
CMIW

[mark stone]

ok. just to confirm that when arrays are declared and instantiated, the length of array in terms of rows and columns cannot change.
right ? they are fixed after they have been declared. Though one can manipulate member values but one cannot add new members.

Originally posted by ravish kumar:
HI
I think
arr[1][0] is equal to null, i.e
arr[1][0] = null;
now array at arr[1][0] does not point to any Array object ... it is null.
hence when we try to allocate anything it throws NPE.
To assign any thing first you have to do :
arr[1] = new int[3];
now you can get arr[1].length
as now it contains refernce to an object and which has a legth of 3
CMIW
CMIW

[Corey McGlone]

just to confirm that when arrays are declared and instantiated, the length of array in terms of rows and columns cannot change.
right ? they are fixed after they have been declared. Though one can manipulate member values but one cannot add new members.

That's right. Once you've declared an array, you can not change its length. You can, however, leave parts of a multi-dimensional array as null and then fill them in when you know the proper length. This isn't really adding new elements to the array, but it does offer some control over the size of the array.
For example, say you're going to read a number of files and store the contents into an array. We'll assume that we can look into one file to get the number of files and names of the files that we'd like to read in. Since it doesn't relate to this question, we'll assume that the file contains a message saying that we'll need to read in 3 files, "foo.txt," "bar.txt," and "baz.txt." Now, to create a properly sized array, you might do something like this:

byte [][] files = new byte[3][];
File f = new File("C:\\foo.txt");
files[0] = new byte[(int)f.length()];
...

Using techniques like this, you can better control the size of your array but, you're right, once an array's length has been set, it can not be changed.
Corey