Yes it does, when using Java 9.

Are you joking? I can't tell.

Just in case you aren't, what do you think is required for a platform to support Hi-DPI displays?

I'm not the OP, but I assume hi-DPI support means that applications scale up their GUI sizes appropriately based on the DPI.

When I use a hi-DPI display (like 4K displays), a lot of old software renders text and widgets too small to see, because they were programmed to display things as so-many pixels high, without concern for DPI. It's frequently a problem with old video games, though it affects regular applications too.

It's more than that, simply pixel-doubling the UI is also terrible. Real hi-DPI support (to me) is actually rendering at hi-dpi.

What difference does it make if the standard resolution is Hi-DPI or not? Can you visually tell the difference between pixel doubling and other methods of designing for Hi-DPI?

For what's it's worth, I've written Java 8 on macOS and it 'just works'. I didn't have to do anything to make it look good in hidpi.

Yes, it works on Mac, but not on Linux...