String toString()

Returns a description of the format exception.

The description always contains the message.

If source is present and is a string, the description will contain (at least a part of) the source. If offset is also provided, the part of the source included will contain that offset, and the offset will be marked.

If the source is a string and it contains a line break before offset, only the line containing offset will be included, and its line number will also be part of the description. Line and character offsets are 1-based.

Source 

String toString() {
  String report = "FormatException";
  if (message != null && "" != message) {
    report = "$report: $message";
  }
  int offset = this.offset;
  if (source is! String) {
    if (offset != null) {
      report += " (at offset $offset)";
    }
    return report;
  }
  if (offset != null && (offset < 0 || offset > source.length)) {
    offset = null;
  }
  // Source is string and offset is null or valid.
  if (offset == null) {
    String source = this.source;
    if (source.length > 78) {
      source = source.substring(0, 75) + "...";
    }
    return "$report\n$source";
  }
  int lineNum = 1;
  int lineStart = 0;
  bool previousCharWasCR = false;
  for (int i = 0; i < offset; i++) {
    int char = source.codeUnitAt(i);
    if (char == 0x0a) {
      if (lineStart != i || !previousCharWasCR) {
        lineNum++;
      }
      lineStart = i + 1;
      previousCharWasCR = false;
    } else if (char == 0x0d) {
      lineNum++;
      lineStart = i + 1;
      previousCharWasCR = true;
    }
  }
  if (lineNum > 1) {
    report += " (at line $lineNum, character ${offset - lineStart + 1})\n";
  } else {
    report += " (at character ${offset + 1})\n";
  }
  int lineEnd = source.length;
  for (int i = offset; i < source.length; i++) {
    int char = source.codeUnitAt(i);
    if (char == 0x0a || char == 0x0d) {
      lineEnd = i;
      break;
    }
  }
  int length = lineEnd - lineStart;
  int start = lineStart;
  int end = lineEnd;
  String prefix = "";
  String postfix = "";
  if (length > 78) {
    // Can't show entire line. Try to anchor at the nearest end, if
    // one is within reach.
    int index = offset - lineStart;
    if (index < 75) {
      end = start + 75;
      postfix = "...";
    } else if (end - offset < 75) {
      start = end - 75;
      prefix = "...";
    } else {
      // Neither end is near, just pick an area around the offset.
      start = offset - 36;
      end = offset + 36;
      prefix = postfix = "...";
    }
  }
  String slice = source.substring(start, end);
  int markOffset = offset - start + prefix.length;
  return "$report$prefix$slice$postfix\n${" " * markOffset}^\n";
}