RFR: 8004518 & 8010122 : Default methods on Map

David Holmes david.holmes at oracle.com
Tue Apr 9 02:22:24 UTC 2013

Hi Mike,

Looking only at Map itself for now.

On 9/04/2013 4:07 AM, Mike Duigou wrote:
> Hello all;
> This is a combined review for the new default methods on the java.util.Map interface being added for the JSR-335 lambda libraries. The reviews are being combined because they share a common unit test.
> http://cr.openjdk.java.net/~mduigou/JDK-8010122/0/webrev/

General style issues:

- spaces after keyword ie "if (x == null)" not "if(x == null)"
- comparisons against constants/null put the constant/null on the right 
ie "if (x == null)" not "if (null == x)"

(where has our style guide gone? I can't find it on internal or external 
wikis :( )

> 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

forEach and replaceAll are very similar in terms of taking and applying 
a "operation" to each entry, yet their descriptions use a completely 
different style. forEach describes thing in general terms while 
replaceAll talks about calling the functions' apply method with the 
current entry's key and value. I would suggest for replaceAll:

"Replaces each entry's value with the result of invoking the given 
function on that entry, in the order entries are returned by an entry 
set iterator, until all entries have been processed or the function 
throws an exception."

This is also makes "replace" the subject rather than "apply".

forEach doesn't declare the IllegalStateException that getKey and 
getValue can throw.

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.

> 8010122: Add atomic operations to Map

Meaning "backport some operations from ConcurrentMap" - they aren't 
actually atomic in Map.

>   getOrDefault()

No comment

>   putIfAbsent()          *

The default implementation throws ConcurrentModificationException but 
this is not declared in the spec.

>   remove(K, V)
>   replace(K, V)
>   replace(K, V, V)

No comments

>   compute()              *
>   merge()                *
>   computeIfAbsent()      *
>   computeIfPresent()     *

The following generally apply to this group of methods.

As Peter already stated the spec:

  * <p>If the function returns {@code null}, the mapping is removed (or
  * remains absent if initially absent).

is somewhat unclear. The parenthesized part is not connected to the 
function returning null or otherwise; as the function won't be called.

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.

? Note computeIfAbsent does not say the same thing.

The @implSpec does not match the actual implementation. It looks to me 
like these implementations are trying to cater for concurrent situations 
- hence the loop. That's okay but then the implSpec should identify that 

980  * be of use when combining multiple mapped values for a key.  For
981  * example. to either create or append a {@code String msg} to a

Period after example should be a comma.


> The * operations treat null values as being absent. (ie. the same as there being no mapping for the specified key).
> The default implementations provided in Map are overridden in HashMap for performance purposes, in Hashtable for atomicity and performance purposes and in Collections for atomicity.
> Mike

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