Sparse Array Detection

[Pho Tek]

Supposed I'm to process an array of class A.
And I want to process it differently if its sparse
or not sparse. ("Sparse" meaning some items are null)
Is there something in the reflection API or normal java.lang
that will tell me whether an array is sparse or not ?
P/S I'd rather not have to write my own routine to iterate
through the whole array.
Thanks
Pho

[kyle amburn]

Hi-
You can use this, it is a little messy but it will work.
import java.util.*;
....
Object[] thisArray = new Object[1000];
...
//populate the array with the items
List tmpList = Arrays.asList(thisArray);
int numItems = tmpList.size();
/*this gives you your number of items in the array. You can then go ahead and compare it to the thisArray.length number to dicide which path you want to take.*/
It would probably be easier if you want to do any amount of operating on arrays to use the java.util.ArrayList class instead.
Kyle

[Pho Tek]

Kyle,
I tried your suggestions but it didn't work.
My code:

import java.util.*;
public class Detect{
  public static void main(String[] args){
    Integer[] ia = new Integer[3];
    ia[0] = null;
    ia[1] = null;
    ia[2] = new Integer(200); 
// The following works only on Object[] arrays not primitive arrays e.g.int[]
    List t = Arrays.asList( ia );
    System.out.println("Number of detected items = " + t.size());
    System.out.println("Number of declared items = " + ia.length);
  }
}

Any other suggestions?
Pho
[This message has been edited by Pho Tek (edited August 04, 2001).]

[William Barnes]

Since there doesn't seem to be any real way of do this you need to start cheating.
The first cheat that I thought of was to save the number of elements in the array in position [ 0 ] of the same array. You would do that as you are loading the array with data.
(Does that make sense?)

[kyle amburn]

Hi Pho-
Sorry about that. I forgot that the list would take the nulls and place them as objects. You can use the method from Christoper or you can try the following. I ran this and got the correct values (i.e. i item, 3 spaces).

import java.util.*;public class Detect{
public static void main(String[] args){
Integer[] ia = new Integer[3];
ia[0] = null;
ia[1] = null;
ia[2] = new Integer(200);
// The following works only on Object[] arrays not primitive arrays e.g.int[]
Vector v = new Vector(Arrays.asList( ia ));
boolean removed;
do {
removed = v.remove(null);
} while(removed);
System.out.println("Number of detected items = " + v.size());
System.out.println("Number of declared items = " + ia.length);
}
}

Kyle

[Colin Kenworthy]

Vector.remove() in a loop ? This is very inefficient - think how many passes through a 1000 element Vector would be needed where just the last 100 are null. The question was could it be done avoiding writing your own routine, so perhaps the answer is simply 'no', and counting the number of null elements might be better in a for loop (you could even 'break' as soon as you found one).

[Pho Tek]

Hey people,
thanks for all your suggestions.
I actually followed the path suggested by Colin and wrote
my own isSparse method by iterating thru the array. It was
then that I was shocked to find that the array that I was
operating on is an array value, an Object instead of Object[]. So I had to use java.lang.reflect.Array to process it. Messy, but I'll live with it for the moment.
Pho