Why not just:
int plusIndex = s.indexOf("+");
if (plusIndex != -1) {
String before = s.substring(0, plusIndex);
// Use before
}
It's not really clear why your original version didn't work, but then you didn't say what actually happened. If you want to split not using regular expressions, I'd personally use Guava:
Iterable<String> bits = Splitter.on('+').split(s);
String firstPart = Iterables.getFirst(bits, "");
If you're going to use split (either the built-in version or Guava) you don't need to check whether it contains + first - if it doesn't there'll only be one result anyway. Obviously there's a question of efficiency, but it's simpler code:
// Calling split unconditionally
String[] parts = s.split("\\+");
s = parts[0];
Note that writing String[] parts is preferred over String parts[] - it's much more idiomatic Java code.