My favorite is immutables (https://immutables.github.io/ https://immutables.github.io/). It's really frictionless and at this point it feels like an extension o

My favorite is immutables (https://immutables.github.io/ https://immutables.github.io/). It's really frictionless and at this point it feels like an extension of the language to me. It also lets you fall back to mutable types when you need them, despite the name.