Should I use string.isEmpty() or "".equals(string)?

The main benefit of "".equals(s) is you don't need the null check (equals will check its argument and return false if it's null), which you seem to not care about. If you're not worried about s being null (or are otherwise checking for it), I would definitely use s.isEmpty(); it shows exactly what you're checking, you care whether or not s is empty, not whether it equals the empty string

String.equals("") is actually a bit slower than just an isEmpty() call. Strings store a count variable initialized in the constructor, since Strings are immutable.

isEmpty() compares the count variable to 0, while equals will check the type, string length, and then iterate over the string for comparison if the sizes match.

So to answer your question, isEmpty() will actually do a lot less! and that's a good thing.

One thing you might want to consider besides the other issues mentioned is that isEmpty() was introduced in 1.6, so if you use it you won't be able to run the code on Java 1.5 or below.