[jfx20] RFR: 8293587: Fix mistakes in FX API docs [v3]

[John Hendrikx]

On Tue, 7 Feb 2023 20:54:20 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> We aren't very consistent on this. Most of the places where we produce warnings, or where we provide verbose output, are done by printing. At a minimum, those should go to `System.err` rather than `System.out`. Unless there is an API whose expected result is to print something to `System.out`, a library really shouldn't print to System.out (as opposed to a test or a demo app, where it's fine). So the only places I would expect to see it in the library -- meaning source code under `modules/javafx.*/src/main/java/` -- would be in examples in javadoc comments.
>> 
>> We could file a second follow-up issue (RFE) to consider changing some of our print statements to use logging.
>
> Some investigation is needed if source can ever be null in such cases.  I think some of these are to inform the programmer (not the user) something unexpected happens, in which case it should be an assert or exception.  If it then ever does happen, it should be fixed.

The drag/drop code is full of what looks like asserts that are just dumped to `System.err`.  It looks like left overs that helped during development of the underlying state machine, but it shouldn't get in such states normally.  When I run the tests, none of the exceptional cases or `System.err` code has any coverage which can be a sign that these cases can't be reached.

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PR: https://git.openjdk.org/jfx/pull/1025