Java 8及更高版本
@沃伦·诺科斯（Warren M. 如果你尝试使用Java 8的新Date and Time API，则可以使用java.time.YearMonthclass。请参阅Oracle教程。
// Get the number of days in that monthYearMonth yearMonthObject = YearMonth.of(1999, 2);int daysInMonth = yearMonthObject.lengthOfMonth(); //28
测试：尝试a年的一个月：
yearMonthObject = YearMonth.of(2000, 2);daysInMonth = yearMonthObject.lengthOfMonth(); //29
Java 7及更早版本
创建日历，设置年份和月份并使用 getActualMaximum
int iYear = 1999;int iMonth = Calendar.FEBRUARY; // 1 (months begin with 0)int iDay = 1;// Create a calendar object and set year and monthCalendar mycal = new GregorianCalendar(iYear, iMonth, iDay);// Get the number of days in that monthint daysInMonth = mycal.getActualMaximum(Calendar.DAY_OF_MONTH); // 28
测试：尝试a年的一个月：
mycal = new GregorianCalendar(2000, Calendar.FEBRUARY, 1);daysInMonth= mycal.getActualMaximum(Calendar.DAY_OF_MONTH); // 29
