if Java had a non-broken string API, even with the current UCS-2 based solution they couldn't do constant-time character lookups.

And even if Java went the extra mile of using characters big enough to encode every code point of a string, constant lookup would still not be possible because users are often interested graphemes, not characters.

Do you know of an example of a well-designed String API? I'd be interested in what trade-offs other language designers made (newer ones, like Rust/Swift/Go/?).

A simple string append can, for instance, require up to 5x the memory consumed by the string.

Worse, this problem will only show up if someone ever enters in a high enough code point. Better hope any user-defined data never gets turned into part of a large string at any point, or you have a potential memory problem down the line.

One language whose name escapes me took an approach where the internal representation is always unicode codepoints, and Strings have methods for projecting those .toUTF8() or whatever to interact with the outside world - I liked that a lot due to its simplicity, anyone knows which language that was?