Java의 일반적인 날짜 연산(값 획득, 변환, 더하기, 빼기, 비교)

//有多个重载方法创建 Calendar 对象
Calendar now = Calendar.getInstance(); //默认
//指定时区和地区，也可以只输入其中一个参数
Calendar now = Calendar.getInstance(timeZone, locale);

int year = now.get(Calendar.YEAR); //，当前年份
int month = now.get(Calendar.MONTH) + ; //，当前月，注意加
int day = now.get(Calendar.DATE); //，当前日Date date = now.getTime(); //直接取得一个 Date 类型的日期

//只设定某个字段的值
 // public final void set(int field, int value)
 now.set(Calendar.YEAR, );
 //设定年月日或者年月日时分或年月日时分秒
 // public final void set(int year, int month, int date[, int hourOfDay, int minute, int second])
 now.set(, , [, , , ]);
 //直接传入一个 Date 类型的日期
 // public final void setTime(Date date)
 now.set(date);

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
 try {
   //日期转字符串
   Calendar calendar = Calendar.getInstance();
   Date date = calendar.getTime();
   String dateStringParse = sdf.format(date);
   //字符串转日期
   String dateString = "-- ::";
   Date dateParse = sdf.parse(dateString);
 } catch (ParseException e) {
   e.printStackTrace();  
 }

//根据现在时间计算
Calendar now = Calendar.getInstance();
now.add(Calendar.YEAR, ); //现在时间的年后
now.add(Calendar.YEAR, -); //现在时间的年前
//根据某个特定的时间 date (Date 型) 计算
Calendar specialDate = Calendar.getInstance();
specialDate.setTime(date); //注意在此处将 specialDate 的值改为特定日期
specialDate.add(Calendar.YEAR, ); //特定时间的年后
specialDate.add(Calendar.YEAR, -); //特定时间的年前

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String dateString = "-- ::";
Calendar calendar = Calendar.getInstance();
long nowDate = calendar.getTime().getTime(); //Date.getTime() 获得毫秒型日期
try {
    long specialDate = sdf.parse(dateString).getTime();
    long betweenDate = (specialDate - nowDate) / ( * * * ); //计算间隔多少天，则除以毫秒到天的转换公式
    System.out.print(betweenDate);
} catch (ParseException e) {
    e.printStackTrace();
}

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String dateString_01 = "2016-01-01 11:11:11";
String dateString_02 = "2016-01-02 11:11:11";
try {
    Date date_01 = sdf.parse(dateString_01);
    Date date_02 = sdf.parse(dateString_02);
    System.out.println(date_01.before(date_02)); //true，当 date_01 小于 date_02 时，为 true，否则为 false
    System.out.println(date_02.after(date_01)); //true，当 date_02 大于 date_01 时，为 true，否则为 false
    System.out.println(date_01.compareTo(date_02)); //-1，当 date_01 小于 date_02 时，为 -1
    System.out.println(date_02.compareTo(date_01)); //1，当 date_02 大于 date_01 时，为 1
    System.out.println(date_02.compareTo(date_02)); //0，当两个日期相等时，为 0
} catch (ParseException e) {
    e.printStackTrace();
}