append(final CharSequence seq, final int startIndex, final int length)
Overrides the method defined by Appendable. However the behavior of the TextStringBuilder implementation is not consistent with the definition in Appendable. As the class defines "that some edge cases, with invalid indices or null input, have been altered - see individual methods" this would not normally be an issue. However as the implementation states that it overrides Appendable the class cannot be used as an Appendable as the last argument is expected to be an end index and not a length.