Question

We’re getting this error

java.lang.NullPointerException
    at java.util.ArrayList.<init>(Unknown Source)
    at de.mystuff.ExtendedArrayList.<init>(ExtendedArrayList.java:38)

where ExtendedArrayList:38 is

new ArrayList(new ArrayCollection<E>(data));

In short: the ArrayList constructor sometimes seems to choke on our home grown Collection implementation ArrayCollection.

I was unable to reproduce it on my computer even with the exact same version that was distributed to our customers.

But i’m not 100% sure that they are using the JRE we included.
So, i googled for some ArrayList.java source code and found openJDK 6b17 which has this

public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    size = elementData.length;
    // c.toArray might (incorrectly) not return Object[] (see 6260652)
    if (elementData.getClass() != Object[].class)
        elementData = Arrays.copyOf(elementData, size, Object[].class);
}

That would make sense, because if there’s no data our ArrayCollection.toArray() returns null. And this constructor looked safe (and worked without exception) for the 1.5.0_09 Sun JDK/JRE implementation we are using.
But openJDK only seems to release for the unix world. Is this code also part of a Windows JRE? And if so, which version?

NB: I know that we have to fix our classes, but i want to make sure i understand the cause of the NullPointerException.

Answer 1

I don’t think you specified exactly which JRE version/vendor you are using, but here is the source for the constructor in question for Sun JDK 1.6.0_17 (if you aren’t already aware, the source for most classes in the java namespace ships with the JDK):

public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    size = elementData.length;
    // c.toArray might (incorrectly) not return Object[] (see 6260652)
    if (elementData.getClass() != Object[].class)
         elementData = Arrays.copyOf(elementData, size, Object[].class);
}

(the comment is the author’s, not mine)

This should pretty clearly show the potential cause of a NPE in this constructor: if c is null or (in your case) if c.toArray() returns null.