Depends on that one line. If the line is readable and concise by itself, the function may not be needed. Simplistic example:

 void printLine() {
 System.out.println();
 }

If the function gives a good name to a line of code containing e.g. a complex, hard to read expression, it is perfectly justified (to me). Contrived example (it is not broken into multiple lines for readability):

 boolean isEligibleForTaxRefund() {
 return taxPayer.isFemale() 
 && (taxPayer.getNumberOfChildren() > 2 
 || (taxPayer.getAge() > 50 && taxPayer.getEmployer().isNonProfit()));
 }