// Figure 9.22public class USAbbrevDate extends Date {	/**	pre:	1 <= m <= 12  	 *			and d is a sensible day for month m	 *			(Note that this method doesn't check date validity)	 *	post:	day == d  and  month == m  and  year == y  	 */	public USAbbrevDate( int d, Month m, int y )  {    	super(d, m, y);	}		/**	post:	result is a string in the form "m/d/y2"	 *			where m is the month number, d is the day number	 *			and y2 is the last two digits of the year.   	 */	public String dateString()   {		return "" + (month.ordinal()+1) + "/" + day + "/" + twoDigitYear();	}}