RFR: 8332895: Support interpolation for backgrounds and borders [v3]

[John Hendrikx]

On Sat, 3 Aug 2024 00:14:20 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> modules/javafx.graphics/src/main/java/com/sun/javafx/util/Utils.java line 197:
>> 
>>> 195:         if (secondList.isEmpty()) {
>>> 196:             return firstList.isEmpty() ? firstList : secondList;
>>> 197:         }
>> 
>> This logic looks odd.  If the second list is empty, we return the first list **if** it is empty (and so empty is returned), otherwise the second list (which is also empty).  So, this is equivalent?
>> Suggestion:
>> 
>>         if (secondList.isEmpty()) {
>>             return List.of();  // suggested here, as the two input lists are not guaranteed to be immutable
>>         }
>
> The point of this excercise is to preferrably return existing instances, even when they are empty. `Border` and `Background` can contain quite a lot of objects, and often only one component is changed in a transition, while all of the rest is left unchanged. We want to elide the allocation of new objects if possible during transitions, which is why most classes check whether the result of an interpolation happens to be the same instance as either the start value or the end value.

But you are checking this with `equals` I hope?  `List.of()` is a constant, it doesn't allocate anything.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1522#discussion_r1702670114