Prototyping in Java is as fast as prototyping in Ruby once you understand how to use the tools equivalent to Pry in Java, and standard types instead of rolling your own types. JIT isso fast today the 20ms required to JIT what you type is immaterial.
Not strictly Java, but Groovy is a great environment to prototype on the JVM. Prototyping was lightning fast and turning it into Java code was pretty quick too. There’s great IDE support for it too (IntelliJ’s Groovy scratches).
These days Kotlin is also pretty good. Perhaps not quite as fast to prototype but there’s little need for converting afterwards.
Yes, add an environment like BeakerX [1] (essentially, Jupyter notebooks) and it's quite amazingly good to figure out your approach even if you intend to write Java.
