Review

class FastDateFormat.NumberRule {
        /**
         * Appends the specified value to the output buffer based on the rule implementation.
         * 
         * @param buffer the output buffer
         * @param value the value to be appended
         */
        void appendTo(StringBuffer buffer, int value);
}

class FastDateFormat.UnpaddedNumberField {
        public final void appendTo(StringBuffer buffer, int value) {
            if (value < 10) {
                buffer.append((char)(value + '0'));
            } else if (value < 100) {
                buffer.append((char)(value / 10 + '0'));
                buffer.append((char)(value % 10 + '0'));
            } else {
                buffer.append(Integer.toString(value));
            }
        }
}

class FastDateFormat.UnpaddedMonthField {
        public final void appendTo(StringBuffer buffer, int value) {
            if (value < 10) {
                buffer.append((char)(value + '0'));
            } else {
                buffer.append((char)(value / 10 + '0'));
                buffer.append((char)(value % 10 + '0'));
            }
        }
}

class FastDateFormat.PaddedNumberField {
        public final void appendTo(StringBuffer buffer, int value) {
            if (value < 100) {
                for (int i = mSize; --i >= 2; ) {
                    buffer.append('0');
                }
                buffer.append((char)(value / 10 + '0'));
                buffer.append((char)(value % 10 + '0'));
            } else {
                int digits;
                if (value < 1000) {
                    digits = 3;
                } else {
                    Validate.isTrue(value > -1, "Negative values should not be possible", value);
                    digits = Integer.toString(value).length();
                }
                for (int i = mSize; --i >= digits; ) {
                    buffer.append('0');
                }
                buffer.append(Integer.toString(value));
            }
        }
}

class FastDateFormat.TwoDigitNumberField {
        public final void appendTo(StringBuffer buffer, int value) {
            if (value < 100) {
                buffer.append((char)(value / 10 + '0'));
                buffer.append((char)(value % 10 + '0'));
            } else {
                buffer.append(Integer.toString(value));
            }
        }
}

class FastDateFormat.TwoDigitYearField {
        public final void appendTo(StringBuffer buffer, int value) {
            buffer.append((char)(value / 10 + '0'));
            buffer.append((char)(value % 10 + '0'));
        }
}

class FastDateFormat.TwoDigitMonthField {
        public final void appendTo(StringBuffer buffer, int value) {
            buffer.append((char)(value / 10 + '0'));
            buffer.append((char)(value % 10 + '0'));
        }
}

class FastDateFormat.TwelveHourField {
        public void appendTo(StringBuffer buffer, int value) {
            mRule.appendTo(buffer, value);
        }
}

class FastDateFormat.TwentyFourHourField {
        public void appendTo(StringBuffer buffer, int value) {
            mRule.appendTo(buffer, value);
        }

}