Numbers starting with 0 formatted into fixed-length string

Simple written 
String.format ( "% 04d", 3 ) 

However, I am surprised that StringBuilder + replace actually higher efficiency

public class Test {
    public static String  format2(int shortval) {
        String formatted = Integer.toString(shortval);
        StringBuilder buf = new StringBuilder("0000");
        buf.replace(4 - formatted.length(), 4, formatted);
        return buf.toString();
    }

    public static void main(String[] args) {
        String s;
        {
            long start = System.nanoTime();
            int cnt = 100_0000;
            while (cnt-- > 0) {
                s = String.format("%04d", 3);
            }
            long end = System.nanoTime();
            System.out.println(end - start);
        }
        {
            long start = System.nanoTime();
            int cnt = 100_0000;
            while (cnt-- > 0) {
                s = format2((short) 3);
            }
            String a ="";
            long end = System.nanoTime();
            System.out.println(end - start);
        }
    }
}