Java strings enjoy heavy optimizations, including SSE4.2 and AVX intrinsics. Implementing your own byte[] wrappers (which Strings are) might be useful however it won't replace the built-in Strings.
In short: a general purpose String substitute in Java would be an extremely poor idea.
Why? IIUC it implies that they _don't_ have their own string implementations. They get the benefit of Java JDK's string implementation (and JVM optimization thereof) for free. If they had their own string implementations, they'd be unable to use this optimization until it's publicly available.
