Java日期类：Date和Calendar的使用
总是使用这两个类，总是需要百度。还不如一次全部整理完。
一、介绍：
Date 类
- Date 表示特定的瞬间，精确到毫秒。
- 在 JDK 1.1 之前，类 Date 有两个其他的函数。它允许把日期解释为年、月、日、小时、分钟和秒值。它也允许格式化和解析日期字符串。不过，这些函数的 API 不易于实现国际化。从 JDK 1.1 开始，应使用 Calendar 类实现日期和时间字段之间转换，使用 DateFormat 类来格式化和解析日期字符串。Date 中的相应方法已废弃（查阅自 API 文档）
Calendar 类
- Calendar 类是一个抽象类，它为特定瞬间与一组诸如 YEAR、MONTH、DAY_OF_MONTH、HOUR 等 日历字段之间的转换提供了一些方法，并为操作日历字段（例如获得下星期的日期）提供了一些方法。
- 瞬间可用毫秒值来表示，它是距历元（即格林威治标准时间 1970 年 1 月 1 日的 00:00:00.000，格里高利历）的偏移量。
简单说就是：java.util.Date 是个日期数据；java.util.Calendar 用于日期相关的计算；
二、使用：
2.1 如何实例化当前时间？
Date date = new Date(); Calendar calendar = Calendar.getInstance(); System.out.println(date.toString()); //输出：Thu Oct 18 13:46:43 CST 2018 System.out.println(calendar.toString()); //输出：java.util.GregorianCalendar[time=1539841603042,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Asia/Shanghai",offset=28800000,dstSavings=0,useDaylight=false,transitions=19,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2018,MONTH=9,WEEK_OF_YEAR=42,WEEK_OF_MONTH=3,DAY_OF_MONTH=18,DAY_OF_YEAR=291,DAY_OF_WEEK=5,DAY_OF_WEEK_IN_MONTH=3,AM_PM=1,HOUR=1,HOUR_OF_DAY=13,MINUTE=46,SECOND=43,MILLISECOND=42,ZONE_OFFSET=28800000,DST_OFFSET=0]
根据输出，我们也可以看出Date和Calendar的区别。
2.2 如何实例化特定时间？
1 //注：mm:ss必须小写，否则无法转换。HH大小写无影响。 2 DateFormat df = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss"); 3 Date date1 = null; 4 try { 5 date1 = df.parse("2017-08-07 11:11:11"); 6 } catch (ParseException e) { 7 e.printStackTrace(); 8 } 9 10 Calendar calendar1 = Calendar.getInstance(); 11 calendar1.setTime(date1);
- 建议从Date使用格式转换，然后赋值（setTime）给Calendar，不然直接设置Calendar用毫秒数（setTimeInMillis）太麻烦。（其实不知道怎么设置….）
- 另外，实例化时的格式：
yyyy-MM-dd hh:mm:ss。mm:ss必须小写，否则无法转换。HH大小写无影响。
2.3 Date和Calendar如何互相转化？
Date date1 = new Date(); Calendar calendar1 = Calendar.getInstance(); calendar1.setTime(date1); Date date = calendar1.getTime();
- setTime接收一个Date类型的参数；
- getTime返回一个Date类型的结果；
主要是通过这个方式实现互相转换。Date类好像没有相关转换的方法
2.4 如何格式化时间？
DateFormat df = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss"); String s = df.format(new Date());
2.5 如何对日期进行操作。即日期加减：年月日以及小时分秒等。
DateFormat df = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss"); Date date1 = new Date(); //输出：2018-10-18 02:18:27 System.out.println(df.format(date1)); Calendar calendar1 = Calendar.getInstance(); calendar1.setTime(date1); calendar1.add(Calendar.MONTH,1); Date date = calendar1.getTime(); //输出：2018-11-18 02:18:27 System.out.println(df.format(date));
abstract void |
add(int field, int amount) 根据日历的规则，为给定的日历字段添加或减去指定的时间量。 |
|---|---|
- field表示时间量：可以为如下：
Calendar.ERA：表示无加减；
Calendar.YEAR：年
Calendar.MONTH：月
Calendar.DATE：日
Calendar.HOUR：小时
Calendar.MINUTE：分钟
Calendar.SECOND：秒
注意：Calendar中的所有常量都可以为参数，不一定局限于上面的。比如Calendar.HOUR_OF_DAY,Calendar.WEEK_OF_MONTH等。不管是什么，只要这个值和周相关就是周（月中的第几周，年中的第几周等），和日相关就是日（一年当中的第几天，一月当中的第几天，一周当中的第几天），和小时相关就是小时
- amount：为整数表示当前时间的基础上加上对应的时间量；为负数表示当前时间的基础上减去对应的时间量。
2.6 其他常用方法
2.6.1 获取当天结束时间：59分59秒0毫秒
private Date getDayEnd() { Calendar cal = new GregorianCalendar(); cal.set(Calendar.HOUR_OF_DAY, 23); cal.set(Calendar.MINUTE, 59); cal.set(Calendar.SECOND, 59); cal.set(Calendar.MILLISECOND, 0); return cal.getTime(); }
2.6.2 日期工具类（DateUtils.java）
package com.dmall.virtual.domain.utils; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import java.text.DateFormat; import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.Calendar; import java.util.Date; public class DateUtils { public static final String MMddHHmmss = "MM-dd HH:mm:ss"; public static final String yyyyMMddHHmmss = "yyyy-MM-dd HH:mm:ss"; public static final String yyyy_MM_dd = "yyyy-MM-dd"; public static final String yyyyMMdd = "yyyy.MM.dd"; public static final String HHmm = "HH:mm"; public static final String MM_dd = "MM-dd"; public static final String yyyy_MM_dd_other = "yyyy-MM-dd"; private static final Logger logger = LoggerFactory.getLogger(DateUtils.class); /** * 根据基础时间(base),单位(unit),偏移量(differ)对时间进行偏移并返回偏移后的时间 * @param base * @param forward * @param unit * @param differ * @return */ public static Date shiftDate(Date base, boolean forward, DateUnit unit, int differ){ if(base == null){ return null; } if(differ <= 0){ return base; } Calendar c = Calendar.getInstance(); c.setTime(base); c.set(unit.getUnit(), c.get(unit.getUnit()) +(forward ? differ : -differ)); return c.getTime(); } public enum DateUnit{ YEAR(Calendar.YEAR), MONTH(Calendar.MONTH), WEEK(Calendar.MONTH), DAY(Calendar.DAY_OF_YEAR), HOUR(Calendar.HOUR_OF_DAY), MINUE(Calendar.MINUTE), SECOND(Calendar.SECOND); private int unit; private DateUnit(int unit){ this.unit = unit; } public int getUnit() { return unit; } public void setUnit(int unit) { this.unit = unit; } } /** * 日期格式化为字符串 * @param date * @param format * @return */ public static String formatDate(Date date, String format) { if (date == null) { return null; } try{ SimpleDateFormat sdf = new SimpleDateFormat(format); return sdf.format(date); }catch(Exception e){ logger.error("日期格式化失败,date={}", date,e); return null; } } /** * 字符串格式化为日期 * @param dateStr * @param format * @return */ public static Date parseDate(String dateStr, String format){ DateFormat formatter = new SimpleDateFormat(format); if(StringUtils.isBlank(dateStr)){ return null; } try { return formatter.parse(dateStr.trim()); } catch (ParseException e) { logger.warn("ParseException日期异常1,参数dateStr={},format={}",new Object[]{dateStr,format,e}); return null; }catch (Exception e) { logger.warn("ParseException日期异常2,参数dateStr={},format={}",new Object[]{dateStr,format,e}); return null; } } /** * 获取当天指定时间（时-分-秒）的日期 * @param hours * @return */ public static Date getSpecialHms(Date date, int hours, int minutes, int seconds) { if(date == null){ return null; } Calendar cal = Calendar.getInstance(); cal.setTime(date); cal.set(Calendar.HOUR_OF_DAY, hours); cal.set(Calendar.MINUTE, minutes); cal.set(Calendar.SECOND, seconds); cal.set(Calendar.MILLISECOND, 0); return cal.getTime(); } /** * 获取指定日期的小时 * @return */ public static int getCurrentHours(Date date){ Calendar calendar = Calendar.getInstance(); calendar.setTime(date); return calendar.get(Calendar.HOUR_OF_DAY); } public static void main(String[] args) { } }
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