Several commercial JDK options have offered compiling to single binary for 20 years, even if out of reach for common folks. GraalVM and OpenJ9 now make it available as free beer as well.

https://docs.oracle.com/en/java/javase/20/docs/api/java.base...

Is that the modern Java equivalent of the single-method interface in Go? I can see its an abstract class - what do you do if you want to implement both Reader and Writer?

You implement both, duh.

Single method interfaces are what lambdas are for.

Its not an interface though:

  public abstract class Reader

Did Java add multiple inheritance or am I missing something?

The interface you'd want to implement is probably https://docs.oracle.com/en/java/javase/20/docs/api/java.base... (and Appendable for the equivalent to Writer, probably AutoCloseable and Flushable as well.)

However, there's lots of code which just takes a Reader/Writer and then you're SOL.

(In general, the I/O interface/class hierarchy in Java is a bit of a mess because a lot of it was retrofitted to maintain binary compatibility. The equivalent stuff in Guava seems a bit saner, but of course not directly usable with all the 3rd party libraries you might want -- it has shims for the most part, though.)

We used ExcelsiorJET to create (pre-compiled) executables for Java (Swing) applications back in 2009 if memory serves me right.

Yes, unfortunely they went out of business, leaving PTC and Aicas as survivors from those days.