As every String concatenation copies the whole string, usually it is preferable to replace it with explicit calls to StringBuilder.append() or StringBuffer.append().

Example:

  String str = "";
  for(int i=0; i<10; i++) {
    str += i;
  }

After the quick-fix is applied:

  String str = "";
  StringBuilder strBuilder = new StringBuilder(str);
  for(int i = 0; i<10; i++) {
    strBuilder.append(i);
  }
  str = strBuilder.toString();

Sometimes, the quick-fixes allow you to convert a String variable to a StringBuilder or introduce a new StringBuilder. Be careful if the original code specially handles the null value, as the replacement may change semantics. If null is possible, null-safe fixes that generate necessary null-checks are suggested. Also, it's not guaranteed that the automatic replacement will always be more performant.