RFR: 8004518 & 8010122 : Default methods on Map

David Holmes david.holmes at oracle.com
Thu Apr 11 05:06:49 UTC 2013

Hi Mike,

Comments inline and trimmed ...

On 11/04/2013 2:41 PM, Mike Duigou wrote:
> On Apr 8 2013, at 19:22 , David Holmes wrote:
>> (where has our style guide gone? I can't find it on internal or external wikis :( )
> This one? http://www.oracle.com/technetwork/java/codeconv-138413.html
> I am unaware of any other guide that's official policy.

I thought we had both OpenJDK style guidelines and internal ones. May be 
confusing with hotspot ones.

>>> 8004518: Add in-place operations to Map
>>>   forEach()
>>>   replaceAll()
>> Both of those contain the boilerplate text:
>>   * <p>The default implementation makes no guarantees about
>>   * synchronization or atomicity properties of this method. Any
>>   * class overriding this method must specify its concurrency
>>   * properties. In particular, all implementations of
>>   * subinterface {@link java.util.concurrent.ConcurrentMap}
>>   * must ensure that this operation is performed atomically.
>> but these methods are not, and can not be, atomic in ConcurrentMap
> I've altered the text to incorporate your suggestions. It now read:
>       * <p>The default implementation makes no guarantees about synchronization
>       * or atomicity properties of this method. Any class which wishes to provide
>       * specific synchronization, atomicity or concurrency behaviour should
>       * override this method.
> I would like to use this same text on the other methods as well.

I'm okay with that, but you should check with whomever thought it 
necessary to call out the ConcurrentMap situation. ConcurrentMap will 
need to override these just to add the spec that they are atomic.

>> forEach doesn't declare the IllegalStateException that getKey and getValue can throw.
> Since forEach isn't supposed to mutate the map this shouldn't happen. It could only happen through concurrent modification. I've added a catch of the IllegalStateException and throw CME with the ISE as the cause to both forEach and replaceAll.

Not sure I see the distinction. These are Map methods so regardless of 
which method is involved you will only get the IllegalStateException if 
there is concurrent modification. Hence to me forEach and replaceAll 
should behave the same way, for example.

>> Some/many of the @throws text has obviously been copied from the Map.Entry methods eg:
>> * @throws ClassCastException if the class of the specified value
>> *         prevents it from being stored in the backing map
>> but when put into Map itself they should not be referring to "the backing map" as there is no "backing map". Further we have inconsistent terminology being used, eg getOrDefault has:
>> * @throws ClassCastException if the key is of an inappropriate type for
>> * this map
>> and then there is a third variant in other methods:
>>   * @throws ClassCastException if the class of the specified key or value
>>   *         prevents it from being stored in this map
>>   *         (<a href="Collection.html#optional-restrictions">optional</a>)
>> These should all have the same basic wording, differing only in key/value.
> agreed. The third variant is now used consistently.

Why do we need the xref to "optional" ? Is throwing CCE optional? Other 
Collection methods don't flag it as optional.

>>>   compute()              *
>>>   merge()                *
>>>   computeIfAbsent()      *
>>>   computeIfPresent()     *
>> The following generally apply to this group of methods.
>> I find the spec for these rather confusing from a concurrency perspective - this non-concurrent interface seems to be trying to say too much about how a concurrent interface should specify behaviour. Why does it need to say:
>>   * In concurrent contexts, the default implementation may retry
>>   * these steps when multiple threads attempt updates.
> For default on Map I am starting to think that throwing that CME rather than looping is the right choice. The retry behaviour seems to be counter the basic behaviour of non-concurrent implementations. The retry behaviour will just hide usage that's otherwise unsafe when used with non-concurrent implementations.
> The concurrent implementations don't use these defaults so there's no problem switching to throwing CME.
> Opinions?

I had assumed the loops only existed because you wanted to use them as 
ConcurrentMap defaults too! If that is not the case then these methods 
should not loop/retry but just throw CME, in my opinion.


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