insert

    /** Inserts str into myLine at position index;
     * no characters from myLine are overwritten.
     * @param str the strng to be inserted
     * @param index the position at which to insert str
     *          Preconidtion: 0 <= index <= myLine.length()
     */
    public void insert(final String str, final int index) {
        if (0 <= index && index <= myLine.length()) {
            String begin = myLine.substring(0, index);
            String end = myLine.substring(index);
            myLine = begin + end;
        }
    }