Related: I'm still frustrated by Java randomly (in a minor release, none the less!) switching to copy-on-substring. If I, a random nobody, had something that had its running time increased by a factor of ~50 (simple recursive-descent-ish parser for coercing tabulated data from one format to another - just call substring to trim off the first token repeatedly), how many dev hours were required overall to fix the results of the change? And there's no simple alternative either or way to preserve the old behavior - the simplest one, rolling your own String class or wrapper, ends up being relatively slow and annoying.

And "all" of this would be solved by having a proper way of doing array slicing - for things like substring's previous worst case (something like a single character being referenced in a substring holding up a gigabyte+-sized string) Java's garbage collector could recognize that the array was only referenced through slices of part of the array, copy that section into another array (updating references to it), and free the large one.

Also, Java's lack of a way to specify that a class is immutable (and that all children classes thereof must also be) is frustrating. Because optimizations like this can and should apply to more than just strings!