Various questions and problems commonly arise with Java Strings. Especially for C programmers, strings in Java have certain quirks, notably the fact that they're immutable (once you've created a String object, you can't change its contents; if you need a mutable string, then you generally have to use some other CharSequence class such as StringBuilder).
At the aforementioned link, I show code examples of some common string functions in Java. Surely, there'll be things I've missed out/forgotton. So please let me know if there's some thing you find yourself needing to do with strings in Java that I haven't mentioned!