# toString method

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.

## Implementation

    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";
    }