c.toArray might (incorrectly) not return Object[] (see 6260652)

Doug Lea dl at cs.oswego.edu
Fri May 22 11:08:47 UTC 2009

David Holmes - Sun Microsystems wrote:
> Florian Weimer said the following on 05/22/09 14:46:
>> It's there, but not in the documentation for toArray():
>> | Note that toArray(new Object[0]) is identical in function to toArray().
>> <http://java.sun.com/j2se/1.4.2/docs/api/java/util/Collection.html#toArray(java.lang.Object[])> 
>> <http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collection.html#toArray(T[])> 
> Ah I see - thanks.
> Okay well the bug is still open. I think the original intent was to 
> change toArray() to match this, but I think it's far too late to change 
> that behaviour now. So the only "fix" is to delete that sentence from 
> the javadoc.

This bug is and its history are well-known to a few of us.
A fix once got vetoed in the approval process out of concern
that existing applications might rely on this incorrect behavior.
However, the bug causes all sorts of collateral damage. For
example, the ArrayList(Collection) constructor includes
     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);

It would be nice to know whether the number of collateral damage
bugs in existing applications is greater than the number of breakages
that would occur if this were fixed. This is not easy to estimate
though because only a few usages of "Object[] a = x.toArray()"
when toArray doesn't return Object[] are actually wrong.


> Were you looking for an actual change in the implementation?
> Note this is just my opinion on this, I'm not a decision maker here :)
> Cheers,
> David

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