String processing in Java
Tim Bray has written On Character Strings, and as usual, it's very worth reading. I think Strings in Java are just about the language's worst feature, so I'm in violent agreement with what he writes.
I remember that IBM's San Francisco (R.I.P.) had its own string class, and the "feature" I hate most about Java is the half-hearted support for the "+"-operator. I don't know how many times I have seen Java code that clearly showed how dangerous the perceived simplicity can be to the performance ...