RFR: 8286269: [java.desktop] Use newer instanceof language feature wherever possible

[Julian Waters]

On Sat, 8 Jan 2022 18:13:45 GMT, SWinxy <duke at openjdk.java.net> wrote:

> The old java.desktop package needs dusting off. It hasn't been kept up to pace with the Java language, but it should. The purpose of this PR is to replace most or all `instanceof` checks with the new `instanceof` with a variable. Some methods that used `instanceof` were also reworked alongside the change (such as `.equals()`) if they were small and they could be clearer.
> There should be no actual change in outcomes from this PR. I have triple checked my work. Some cases are kinda 'wut?'.
> This is also a large change. When pattern matching for switch arrives, many methods can be reworked to take advantage of it (when it comes.. eventually ;w;).

Try to merge instead of rebasing and force-pushing, it helps with later reviews. I can try to help you get a JBS issue for this too, since you need one for the PR to be accepted (No guarantees though)

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PR: https://git.openjdk.java.net/jdk/pull/7000