I worked on a Java desktop app, that had a highly complex UI written in Swing, for ~14 years. We only officially targeted Windows as a platform. For a few years I switched to a Linux development machine. The only code change required was for our third party license dongle - to reference the correct native library needed for machine fingerprinting. We also ran it on another developers Mac just to see if we could - again we only had to comment out the licensing code and it works flawlessly.

So from that experience I would say Java definitely lived up to Write Once Run Everywhere. FWIW we used the Nimbus Look and Feel. Other LAFs may not have had as much polish applied.

It did, once you finished debugging everywhere.

That aside; these days, it actually more or less works.

I avoid it as far as I can after having built a couple of systems in the language. For all its faults, Go is still a much more pleasant language to solve the same kinds of problems in.

But my latest project needed multi-platform printing, so I cobbled together a tiny Java program using the print API that I run in the background and pipe data to, and it just works.

all those software already have already proven that.