description

# toString method Null safety

String toString ()
override

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";
      Object? message = this.message;
      if (message != null && "" != message) {
        report = "$report: $message";
      }
      int? offset = this.offset;
      Object? source = this.source;
      if (source is String) {
        if (offset != null && (offset < 0 || offset > source.length)) {
          offset = null;
        }
        // Source is string and offset is null or valid.
        if (offset == null) {
          if (source.length > 78) {
            source = source.substring(0, 75) + "...";
          }
          return "$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";
      } else {
        // The source is not a string.
        if (offset != null) {
          report += " (at offset $offset)";
        }
        return report;
      }
    }