Repeating strings with a separator in Java

The problem of joining strings with a separator is a very common one in programming, but is surprising the number of solutions that different programmers find to that simple task.
Recently I had to repeat a given string ("?" for example) a given number of times but adding a separator between each string ("," for example). I searched for that code in other places of the project and I found many ways, including these:

if (n > 0) {
 for (int i = 1; i < n; i++) {
  buf.append("?,");
 }
 buf.append("?");
}

if (n > 0) {
 for (int i = 0; i < n; i++) {
  buf.append("?,");
 }
 buf.setLength(buf.length() - 1);
}

for (int i = 0; i < n; i++) {
 buf.append("?");
 if (i < n - 1) {
  buf.append(",");
 }
}

boolean first = true;
for (int i = 0; i < n; i++) {
 if (first) {
  first = false;
 } else {
  buf.append(",");
 }
 buf.append("?");
}

To stop this madness, I decided to create a method for reducing all those variations to simpler and less error-prone calls:

/**
 * Appends a string to a buffer a specified number of times,
 * optionally inserting another string as a separator.
 *
 * @param sb The buffer that will have the string appended, if null a new one is created.
 * @param str The string to append.
 * @param n The number of times that the string will be appended.
 * @param sep An optional separator appended between two strings.
 *
 * @return The specified buffer with the string appended or a new one if it was null.
 */
public static StringBuilder repeatAppend(StringBuilder sb, String str, int n, String sep) {
  if (sb == null) {
    sb = new StringBuilder();
  }
  if (n > 0) {
    sb.append(str);
    if (sep != null) {
      for (int i = 1; i < n; i++) {
        sb.append(sep).append(str);
      }
    } else {
      for (int i = 1; i < n; i++) {
        sb.append(str);
      }
    }
  }
  return sb;
}

Be free to use it in your project and happy coding!

Update: Apache Commons StringUtils.repeat(str, sep, n) and Java Collections.nCopies(n, str) with String.join(sep, collection) are alternatives to this method, but our method allows adding to an existing StringBuilder and the others need to create it and destroy it for that. Added return sb; to allow an inline use.