Calendar最大最难理解的误区

以你之姓@ 2023-06-29 02:24 26阅读 0赞

## Calendar最大最难理解的误区，通过代码输出得到真正的结果，恍然大悟 ##

### 代码演示： ###

package com.liangrui.javebase;
    
    import java.util.Calendar;
    import java.util.Date;
    import java.util.GregorianCalendar;
    
    /** * @author liangrui * @date 2020年1月10日 上午1:20:30 * @version 1.0.0 * @description Calendar最大最难理解的误区，通过代码输出得到真正的结果，恍然大悟 * cal.get(Calendar.DAY_OF_MONTH) 获取真实日期在当月的日期，括号里面的Calendar.DAY_OF_MONTH是为了“告诉”get方法取出“cal真实日期”在该月的日期 * cal.DAY_OF_MONTH 和Calendar.DAY_OF_MONTH是一样的，状态常量5，但是静态变量这样操作是不合法的 * Calendar.DAY_OF_MONTH 表示一种状态常量，为了告诉方法应该使用哪种属性 */
    public class CalendarBiggestMistake { 
    
    
    	public static void main(String[] args) { 
    		
    		Date date=new Date();
    		Calendar cal=new GregorianCalendar();
    		cal.setTime(date);
    		
    		System.out.println(cal.get(Calendar.DAY_OF_MONTH));//输出结果：10
    		System.out.println(cal.DAY_OF_MONTH);//输出结果：5
    		System.out.println(Calendar.DAY_OF_MONTH);//输出结果：5
    
    	}
    
    }

## 来看看java中Calendar源码各个静态常量就能知晓了 ##

/** * Field number for <code>get</code> and <code>set</code> indicating the * year. This is a calendar-specific value; see subclass documentation. */
        public final static int YEAR = 1;
    
        /** * Field number for <code>get</code> and <code>set</code> indicating the * month. This is a calendar-specific value. The first month of * the year in the Gregorian and Julian calendars is * <code>JANUARY</code> which is 0; the last depends on the number * of months in a year. * * @see #JANUARY * @see #FEBRUARY * @see #MARCH * @see #APRIL * @see #MAY * @see #JUNE * @see #JULY * @see #AUGUST * @see #SEPTEMBER * @see #OCTOBER * @see #NOVEMBER * @see #DECEMBER * @see #UNDECIMBER */
        public final static int MONTH = 2;
    
        /** * Field number for <code>get</code> and <code>set</code> indicating the * week number within the current year. The first week of the year, as * defined by <code>getFirstDayOfWeek()</code> and * <code>getMinimalDaysInFirstWeek()</code>, has value 1. Subclasses define * the value of <code>WEEK_OF_YEAR</code> for days before the first week of * the year. * * @see #getFirstDayOfWeek * @see #getMinimalDaysInFirstWeek */
        public final static int WEEK_OF_YEAR = 3;
    
        /** * Field number for <code>get</code> and <code>set</code> indicating the * week number within the current month. The first week of the month, as * defined by <code>getFirstDayOfWeek()</code> and * <code>getMinimalDaysInFirstWeek()</code>, has value 1. Subclasses define * the value of <code>WEEK_OF_MONTH</code> for days before the first week of * the month. * * @see #getFirstDayOfWeek * @see #getMinimalDaysInFirstWeek */
        public final static int WEEK_OF_MONTH = 4;
    
        /** * Field number for <code>get</code> and <code>set</code> indicating the * day of the month. This is a synonym for <code>DAY_OF_MONTH</code>. * The first day of the month has value 1. * * @see #DAY_OF_MONTH */
        public final static int DATE = 5;
    
        /** * Field number for <code>get</code> and <code>set</code> indicating the * day of the month. This is a synonym for <code>DATE</code>. * The first day of the month has value 1. * * @see #DATE */
        public final static int DAY_OF_MONTH = 5;
    
        /** * Field number for <code>get</code> and <code>set</code> indicating the day * number within the current year. The first day of the year has value 1. */
        public final static int DAY_OF_YEAR = 6;
    
        /** * Field number for <code>get</code> and <code>set</code> indicating the day * of the week. This field takes values <code>SUNDAY</code>, * <code>MONDAY</code>, <code>TUESDAY</code>, <code>WEDNESDAY</code>, * <code>THURSDAY</code>, <code>FRIDAY</code>, and <code>SATURDAY</code>. * * @see #SUNDAY * @see #MONDAY * @see #TUESDAY * @see #WEDNESDAY * @see #THURSDAY * @see #FRIDAY * @see #SATURDAY */
        public final static int DAY_OF_WEEK = 7;

还有很多就不一一列举了

## 总结： ##

**cal.get(Calendar.DAY\_OF\_MONTH)**

*  获取真实日期在当月的日期，括号里面的Calendar.DAY\_OF\_MONTH是为了 “**告诉get方法**” 取出 “**cal真实日期**” 在“**该月的日期**”。

**Calendar.DAY\_OF\_MONTH**

*  表示一种**状态常量**，为了**告诉方法应该使用哪种属性**，只是起到**功能作用**

**cal.DAY\_OF\_MONTH**

*  和Calendar.DAY\_OF\_MONTH是一样的，状态常量5，**但是静态变量这样操作是不合法的**