I came accross another interesting finding. It looks like the CharSequence/Appendable interfaces/traits are not systematically used in many places of the javalib. I think it is inherited from initial Java design (see: https://stackoverflow.com/questions/5949926/what-is-the-difference-between-append-and-write-methods-of-java-io-writer).

The good news is that in several places CharSequence could be used instead of (or additionally to) String. This would avoid some possible extra allocations.

I have a simple example: printing to a PrintStream a StringBuilder. PrintStream implements a method like print(obj: AnyRef) that can thus accept a StringBuilder:

So what is my point? If PrintStream was modified internally to call encoder.append(csq: CharSequence), it would avoid the StringBuilder.toString trip and thus save additional String object allocation.

Remark: this problem is even more important because of the way AbstractStringBuilder.toString currently works, and as described in this other issue: #2905
It means that currently it will not only create a new String object instance, but it will also copy the underlying Array systematically.