RFR 7065380 : Allow Collections.sort to sort Collections.singletonList() result

Joe Darcy joe.darcy at oracle.com
Fri Mar 2 00:22:58 UTC 2012

Hi Mike,

The main body of the javadoc of method does state

  176      * <p>The specified list must be modifiable, but need not be 

so I agree that a small javadoc update is needed to support this 
reasonable expansion of behavior.

A further expansion would be "The list must be modifiable if not already 
sorted," but I haven't checked to see if that would require further 
implementation changes.


On 03/01/2012 02:02 PM, Mike Duigou wrote:
> I always read @throws declarations as "if thrown then description was the cause" rather than "will be thrown if description". A minor difference in interpretation that can sometimes be useful.
> For this particular case the restriction on sort() seems to serve only to blunt the usefulness of Collections.emptyList and singletonList(). I'd prefer to bend the rules slightly rather than requiring developers to use empty and singleton ArrayLists.
> Mike
> On Mar 1 2012, at 13:29 , Colin Decker wrote:
>> Doesn't this break the contract of the method? It specifies that it throws UnsupportedOperationException if the specified list's list-iterator does not support the set operation. Its Javadoc body also states that the list must be modifiable. (Though sort() already succeeds for an emptyList() despite it not supporting set.)  It seems to me that this just hides programmer error. Anyone who passes an unmodifiable singleton list to sort() is treating it as a modifiable list in their code. Rather than succeeding despite that, I'd think it might be preferable for the call to fail so the programmer is alerted to that error in their assumptions so they can correct it before it bites them elsewhere.
>> -- 
>> Colin
>> 5
>> On Thu, Mar 1, 2012 at 2:40 PM, Mike Duigou<mike.duigou at oracle.com>  wrote:
>> Hello all;
>> Currently Collections.sort() refuses to sort the lists which result from calling Collections.singletonList(). This makes some sense because the singleton lists are immutable but they are also alway sorted.
>> This patch allows Collections.sort() to be used with empty and singleton lists of all types. A short circuit return is provided for lists of length 0 and 1 as they are already sorted.
>> WEBREV @ http://cr.openjdk.java.net/~mduigou/7065380/0/webrev/
>> For the unit test ignore the diffs and view the "New" file--webrev doesn't understand "hg copy".
>> Thanks,
>> Mike

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