RFR: 8305072: Win32ShellFolder2.compareTo is inconsistent [v2]

[Alexey Ivanov]

On Thu, 14 Mar 2024 16:49:39 GMT, Alexey Ivanov <aivanov at openjdk.org> wrote:

>> src/java.desktop/windows/classes/sun/awt/shell/Win32ShellFolderManager2.java line 525:
>> 
>>> 523:         boolean special2 = sf2.isSpecial();
>>> 524: 
>>> 525:         if (special1 && special2) {
>> 
>> Could you please say something on how/why this change solves it ?
>
> It is well described in a [JBS comment](https://bugs.openjdk.org/browse/JDK-8305072?focusedId=14610400&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14610400) by the submitter.
> 
> Let's consider the failing case where:
> 
> a = C:\Users<user>\Documents(true)
> b = C:\Users<user>(false)
> c = C:\Users<user>\Documents(false)
> 
> The value in parentheses is `isSpecial()` flag.
> 
> When `a` and `c` are compared, `special1 = true` and `special2 = false`, therefore the code flow enters this `if`-block. Because the `equals` method doesn't take into account the `isSpecial` flag, both `i1` and `i2` are 0.
> 
> https://github.com/openjdk/jdk/blob/7bccc847fe9e8c5e640d8fa9c93ce32563a0ccfa/src/java.desktop/windows/classes/sun/awt/shell/Win32ShellFolderManager2.java#L534-L535
> 
> Thus, `a` and `c` are considered equal, which breaks the general contract of [`Comparable.compareTo`](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Comparable.html#compareTo(T)).
> 
> If you look below this `if`-statement:
> 
> https://github.com/openjdk/jdk/blob/7bccc847fe9e8c5e640d8fa9c93ce32563a0ccfa/src/java.desktop/windows/classes/sun/awt/shell/Win32ShellFolderManager2.java#L545-L550
> 
> The comment and the code say that any special folder sorts above any regular folder. Therefore the code shouldn't enter that `if`-block above if only one of the folders is *special* — the order is already well-defined for such case.
> 
> The purpose of the `if`-block is to make some of the special folders even more special. In other words, it ensures the following special folders—`getPersonal`, `getDesktop`, `getDrives`, `getNetwork`—are at the top of the list, above any other special folders.

So, changing the condition in the `if`-block from `special1 || special2` to `special1 && special2` makes the sorting order consistent because the relation is now *transitive*:


a < b & b < c therefore a < c

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/18126#discussion_r1525214351