I personally know of an HFT firm that used Java approximately a decade ago. My guess would be they're still using it today given Java performance has only improved since then.
it doesn't mean Java is optimal or close to optimal choice.
Amount of extra effort they do to achieve goals could be significant.
Optimal in what sense? In the java shops I've worked at it's usually viewed as a pretty optimal situation to have everything in one language. This makes code reuse, packaging, deployment, etc much simpler.
In terms of speed, memory usage, runtime characteristics... sure there are better options. But if java is good enough, or can be made good enough by writing the code correctly, why add another toolchain?
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