ways to deal with null poingt exceptions

[Scott Pike]

I'm doing a program for class. getting an average from acollection. We are using test Junits. one of the test involves null enteret into the collection. this causes a nullpointexception. I tried to use the try/catch thing to catch the null so it dosent crash the program but it dosen't work. Is there any way to deal with this thing so it will pass the Junit test? I tried couple things but it didn't work out. Heres the code for my class and the Junit test class.
Here is the error I get when I run the test.
:java.lang.nullpointerException
on lines 44 and 40.


Junit

package CIT310;
 
import static org.junit.Assert.*;
 
import java.util.ArrayList;
 
import org.junit.Test;
 
public class StatisticsTest {
 
@Test
public void testAverage() {
ArrayList<Number> aCollection = new ArrayList<Number>();
aCollection.add(5);
aCollection.add(10);
aCollection.add(-5);
double expected = 10.0/3.0;
Double anAverage = Statistics.getAverage(aCollection);
assertEquals(expected, anAverage.doubleValue(),.0001);


aCollection.clear();
/*
 * checking handling of overflow condition
 */
aCollection.add(4);
aCollection.add(Double.MAX_VALUE);

anAverage = Statistics.getAverage(aCollection);
assertNull(anAverage);

aCollection.clear();
/*
 * checking that null values in the collection are ignored
 */
aCollection.add(null);
aCollection.add(5);
aCollection.add(10);

anAverage = Statistics.getAverage(aCollection);

assertEquals(7.5, anAverage);
/* ch
* checking that Non
*/
ArrayList aList = new ArrayList();
aList.add(new String("hello"));
aList.add(5);

anAverage = Statistics.getAverage(aList);

assertEquals(5.0, anAverage, 0);




}
 
}

My Program Statistics class.

package CIT310;
 
 
import java.util.Collection;
import java.util.Iterator;
public class Statistics {
 
public static Double getAverage(Collection acollection) {
// TODO Auto-generated method stub
double anAverage = 0;
Iterator it=acollection.iterator();
double sum = 0.0;
double spaceAmount = 0;
Number aNumber = 0;
double spaceLeft = 0;
 int size = 0;
//while loop passes through collection 
while (it.hasNext())
{

try { aNumber =  (Number)it.next();
}catch (NullPointerException e){}
Number nullTest = aNumber;
if (null == nullTest)
{
it.remove();
size--;
}
else

spaceAmount = aNumber.doubleValue();
spaceLeft = Double.MAX_VALUE - sum;

if (spaceLeft <= spaceAmount)
{
System.out.println("There is an error, a double digit has caused" +
"an overflow");
return null;
}
else
{
 size++;
 sum = sum +aNumber.doubleValue();
}


}
 



 anAverage = sum/acollection.size();
return  anAverage;
}
}

[ September 22, 2007: Message edited by: Jim Yingst ]

[Stan James]

What did "didn't work" look like? An exception or incorrect result?

Try moving more of your code inside the try and catch blocks. Put all the "happy path" stuff in the try, all the error code in the catch. See if that clears things up. It should let you remove a bunch of code!

And try the CODE button when you post code. That will preserve your indenting and make things easier on readers.
[ September 22, 2007: Message edited by: Stan James ]

[v ray]

well,, obviously the first option is to have a check for null before adding to the collection.

Also, I dont get this part of the code:

if (null == nullTest)
{
it.remove();
size--;
}

it.remove()? whats the point in removing from the iterator? Shouldn't you do a "acollection.remove()"?

The logic is right though, iterate through the collection, remove all nulls, and then try to do any operation on it so that a nullPointerException is not thrown.

[Nicholas Jordan]

It's horrible. Nearly everyone thinks aforehand that if it is later, then it is greater. You hear comments sometimes that Hashtable should not be used, explaing that it was written earlier. Well, I got news for ya: That would be like saying Object should not be used because it was written in an earlier time ~ another era, actually, and has been superceded by the Bloat-O-Matic Code Hog that swamps all known page-demand algorithms.

The answer to your queston is best approached by a tradional idiom that I saw recently and have now learned to place in my code, instinctively.

if(s == null )handle;
That prevents exceptions from originating at that point in the code.

Now, if you get one [an exception] you handle it in the catch clause. Say for an example a String is held in your collection. If you encounter a null, just set that string to a new empty string as:

if(s == null )s = new String("");

Or call next on the iterator or something.

You handle exceptions in a catch clause.
[ September 22, 2007: Message edited by: Nicholas Jordan ]

[v ray]

if (null == nullTest)
{
it.remove();
size--;
}

Ah! Such a simple mistake , I was reading something about iterators today and I remembered something really simple.
If you want to remove something while iterating through a collection, you have to iterate through the next element and then call it.remove().

ie, the above code would be:

if (null == nullTest)
{
it.next();
it.remove();
size--;
}

[Nicholas Jordan]

Not really sure, but it is going to go something like that. More experience coders may know the correct way or I can look it up for you easily if you want, but in general all ops on an Iterator pass through to the underlying collection: If you called next, then remove, you will likely remove the next element, not the one it was just pointing at. Collections behave somewhat differently, from one to the next - but these are underlying basic mechanics of computer science rising like an air bubble in a coffee pot ---> once they get to the top, you don't see anything.