【Android】日期的存取/運算

在 Android 中，要對日期做運算，要用到 Calendar 這個類別，而此類別位於 java.util.Calendar，可以看到是以 java 開頭的，所以也適用於 java 程式中。

首先宣告建立 Calendar 實體

Calendar  mCal = Calendar.getInstance();

物件一生成時, 便已存有目前的日期及時間.

// 本月第一天
mCal.set(Calendar.DAY_OF_MONTH,1);

// 本月最後一天
mCal.set(Calendar.DAY_OF_MONTH, mCal.getActualMaximum(Calendar.DAY_OF_MONTH)); 

// 到前 x 個月
mCal.add(Calendar.MONTH, -x);

// 到後 x 個月
mCal.add(Calendar.MONTH, x);

// 到前 x 天
mCal.add(Calendar.DATE, -x);

// 到後 x 天
mCal.add(Calendar.DATE, x);

// 當月天數
mCal.getActualMaximum(Calendar.DAY_OF_MONTH);

// 直接設定年份為 1967 年
mCal.set(Calendar.YEAR, 1967);

// 直接設定月份為 2 月，注意月份是從 0 起頭
mCal.set(Calendar.MONTH, 1);

// 直接設定時間的時欄位為下午 1 點
mCal.set(Calendar.HOUR, 13);

// 直接設定時間的分欄位為 22 分
mCal.set(Calendar.MINUTE, 22);

// 直接設定時間的秒欄位為 33 秒
mCal.set(Calendar.SECOND, 33);

// 判斷現在為 AM 或 PM，0: AM，1: PM
mCal.get(Calendar.AM_PM);

// 判斷是否為閏年，先將月份移至 2 月
// 再看 2 月有幾天，29 天為閏年，其他則為平年
mCal.set(Calendar.MONTH, 1);
if(mCal.getActualMaximum(Calendar.DAY_OF_MONTH)==29)
       setTitle("今年為閏年");
else
       setTitle("今年不是閏年,是平年");

// 今天是星期幾
mCal.get(Calendar.DAY_OF_WEEK);

// 查日曆日, 即今天是今年的第幾天
mCal.get(Calendar.DAY_OF_YEAR);

// 本週是今年第幾週
mCal.get(Calendar.WEEK_OF_YEAR);
// 設定每週首日為星期日
mCal.setFirstDayOfWeek(Calendar.SUNDAY);

// 設定每週首日為星期一
mCal.setFirstDayOfWeek(Calendar.MONDAY);

// 查每週首日，1: 星期日，2: 星期一
mCal.getFirstDayOfWeek();

從上面的例子可以發現, 一月整數值是 0，星期日是 1，各日期單位的第一順位(起始) 的整數值都不相同，所以善用 Calendar 類別常數就不需辛苦死記了。



下列是 Calendar 類別常數, 記得使用時要以 Calendar 開頭，例如上述範例的 Calendar.SUNDAY，Calendar.DAY_OF_WEEK......等  ----

int	ALL_STYLES	Requests both SHORT and LONG styles in the map returned by getDisplayNames(int, int, Locale).
int	AM	Value of the AM_PM field indicating the period of the day from midnight to just before noon.
int	AM_PM	Field number for get and set indicating whether the HOUR is before or after noon.
int	APRIL	Value of the MONTH field indicating the fourth month of the year.
int	AUGUST	Value of the MONTH field indicating the eighth month of the year.
int	DATE	Field number for get and set indicating the day of the month.
int	DAY_OF_MONTH	Field number for get and set indicating the day of the month.
int	DAY_OF_WEEK	Field number for get and set indicating the day of the week.
int	DAY_OF_WEEK_IN_MONTH	Field number for get and set indicating the ordinal number of the day of the week within the current month.
int	DAY_OF_YEAR	Field number for get and set indicating the day number within the current year.
int	DECEMBER	Value of the MONTH field indicating the twelfth month of the year.
int	DST_OFFSET	Field number for get and set indicating the daylight savings offset in milliseconds.
int	ERA	Field number for get and set indicating the era, e.g., AD or BC in the Julian calendar.
int	FEBRUARY	Value of the MONTH field indicating the second month of the year.
int	FIELD_COUNT	This is the total number of fields in this calendar.
int	FRIDAY	Value of the DAY_OF_WEEK field indicating Friday.
int	HOUR	Field number for get and set indicating the hour of the morning or afternoon.
int	HOUR_OF_DAY	Field number for get and set indicating the hour of the day.
int	JANUARY	Value of the MONTH field indicating the first month of the year.
int	JULY	Value of the MONTH field indicating the seventh month of the year.
int	JUNE	Value of the MONTH field indicating the sixth month of the year.
int	LONG	Requests long names (such as "January") from getDisplayName(int, int, Locale) or getDisplayNames(int, int, Locale).
int	MARCH	Value of the MONTH field indicating the third month of the year.
int	MAY	Value of the MONTH field indicating the fifth month of the year.
int	MILLISECOND	Field number for get and set indicating the millisecond within the second.
int	MINUTE	Field number for get and set indicating the minute within the hour.
int	MONDAY	Value of the DAY_OF_WEEK field indicating Monday.
int	MONTH	Field number for get and set indicating the month.
int	NOVEMBER	Value of the MONTH field indicating the eleventh month of the year.
int	OCTOBER	Value of the MONTH field indicating the tenth month of the year.
int	PM	Value of the AM_PM field indicating the period of the day from noon to just before midnight.
int	SATURDAY	Value of the DAY_OF_WEEK field indicating Saturday.
int	SECOND	Field number for get and set indicating the second within the minute.
int	SEPTEMBER	Value of the MONTH field indicating the ninth month of the year.
int	SHORT	Requests short names (such as "Jan") from getDisplayName(int, int, Locale) or getDisplayNames(int, int, Locale).
int	SUNDAY	Value of the DAY_OF_WEEK field indicating Sunday.
int	THURSDAY	Value of the DAY_OF_WEEK field indicating Thursday.
int	TUESDAY	Value of the DAY_OF_WEEK field indicating Tuesday.
int	UNDECIMBER	Value of the MONTH field indicating the thirteenth month of the year.
int	WEDNESDAY	Value of the DAY_OF_WEEK field indicating Wednesday.
int	WEEK_OF_MONTH	Field number for get and set indicating the week number within the current month.
int	WEEK_OF_YEAR	Field number for get and set indicating the week number within the current year.
int	YEAR	Field number for get and set indicating the year.
int	ZONE_OFFSET	Field number for get and set indicating the raw offset from GMT in milliseconds.