What does String.compareTo return, and how does it order strings?

compareTo does a lexicographic (dictionary) comparison character by character using Unicode values.

It returns a negative int if this string sorts before the argument, zero if equal, and positive if after. If one is a prefix of the other, the shorter sorts first and the return value is the length difference. This is the natural ordering used by Collections.sort, TreeSet, and TreeMap; a Comparator lets you define a different ordering (e.g. by length, or case-insensitive).