masking a creditcard number in java

I tried to mask the characters in a creditcard number string using character 'X'.I wrote two functions as below .The second function uses commons.lang.StringUtils class .I tried to find the time it takes in both cases

public static String maskCCNumber(String ccnum){
        long starttime = System.currentTimeMillis();
        int total = ccnum.length();
        int startlen=4,endlen = 4;
        int masklen = total-(startlen + endlen) ;
        StringBuffer maskedbuf = new StringBuffer(ccnum.substring(0,startlen));
        for(int i=0;i<masklen;i++) {
            maskedbuf.append('X');
        }
        maskedbuf.append(ccnum.substring(startlen+masklen, total));
        String masked = maskedbuf.toString();
        long endtime = System.currentTimeMillis();
        System.out.println("maskCCNumber:="+masked+" of :"+masked.length()+" size");
        System.out.println("using StringBuffer="+ (endtime-starttime)+" millis");
        return masked;
    }

    public static String maskCCNumberCommons(String ccnum){
        long starttime = System.currentTimeMillis();
        int total = ccnum.length();
        int startlen=4,endlen = 4;
        int masklen = total-(startlen + endlen) ;
        String start = ccnum.substring(0,startlen);
        String end = ccnum.substring(startlen+masklen, total);
        String padded = StringUtils.rightPad(start, startlen+masklen,'X'); 
        String masked = padded.concat(end);
        long endtime = System.currentTimeMillis();
        System.out.println("maskCCNumber:="+masked+" of :"+masked.length()+" size");
        System.out.println("using Stringutils="+(endtime-starttime)+" millis");
        return masked;
    }

public static void ccNumberMaskingDemo() {
   String mcard1="5555555555554444";
   maskCCNumber(mcard1);
   maskCCNumberCommons(mcard1);
}

When I ran this ,I got this result

maskCCNumber:=5555XXXXXXXX4444 of :16 size
using StringBuffer=0 millis
maskCCNumber:=5555XXXXXXXX4444 of :16 size
using Stringutils=25 millis

I can't understand why commons.StringUtils is taking more time than the for loop+StringBuffer in the first function.Obviously I am using the api ,the wrong way..

Can someone advise how to use this api correctly, in this case?

Firstly, if you make measurements of such a short-running code, you often do not get accurate results due to the minimal timing resolution your CPU/library/whatever provides (which means you usually get to see 0ms or the same small value over and over).

Second and more importantly, do not optimize this! "Premature optimization is the root of all evil" and in a case where you have only a few ms that you want to optimize the effort is thoroughly wasted. You would have to mask millions of credit cards before you should even remotely think about optimizing this simple mask method.