hg: lambda/lambda/jdk: - ensure null values are retained

[Remi Forax]

On 12/03/2012 04:59 PM, paul.sandoz at oracle.com wrote:
> Changeset: 8d6fc90e7bbf
> Author:    psandoz
> Date:      2012-12-03 16:57 +0100
> URL:       http://hg.openjdk.java.net/lambda/lambda/jdk/rev/8d6fc90e7bbf
>
> - ensure null values are retained
> - add very simple data provider for combination of numbers and null values
>
> ! src/share/classes/java/util/stream/op/UniqOp.java
> ! test-ng/tests/org/openjdk/tests/java/util/stream/StreamTestDataProvider.java
> ! test-ng/tests/org/openjdk/tests/java/util/stream/op/UniqOpTest.java
>
>

Hi Paul,
that's not the good way(TM) to handle null.

A better approach is to use the Null Object design pattern, i.e.
having a constant final field Object with a known object and each time
a null is sent, null is replaced by the Object and each time the element
need to be sent, if element value is equals to the known Object, null is 
sent.
You can take a look to IdentityHashMap.maskNull/unmaskNull [1] which already
implements this pattern.

This can be done locally to each ops or globally for the whole pipeline,
the former solution is enough in my opinion.

The null object pattern is better because it adds only two checks on 
variable that are already in register
if the code never see a null (the usual case). Your solution require to 
load/store value from the RAM
because the VM will not able to optimize this code (inlining is often 
defeated due to the fact that the call
between ops is a virtual call).

cheers,
Rémi

[1] 
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/df5619994dc3/src/share/classes/java/util/IdentityHashMap.java