The method seems to be building a String using concatenation in a loop. In each iteration, the String is converted to a StringBuffer/StringBuilder, appended to, and converted back to a String. This can lead to a cost quadratic in the number of iterations, as the growing string is recopied in each iteration. 

Better performance can be obtained by using a StringBuffer (or StringBuilder in Java 5) explicitly.

 For example:

// This is bad
String s = "";
for (int i = 0; i < field.length; ++i) {
    s = s + field[i];
}

// This is better
StringBuffer buf = new StringBuffer();
for (int i = 0; i < field.length; ++i) {
    buf.append(field[i]);
}
String s = buf.toString();