How to use a string in an if statement? [duplicate]

This question already has an answer here:

• bytes of a string in java? 7 answers
• How to correctly compute the length of a String in Java? 5 answers

I need to make it so that less than 3 characters results in an error message, and 3 or more prints the statement. So for the beginning of the if statement I put if(refNumber > " ") and so on, however, the string comes up as an error.