有人请解释Calendar.get(cal.DAY_OF_WEEK)如何提供正确的输出，而cal.DAY_OF_WEEK是默认值且未修改吗? [英] Someone please explain how Calendar.get(cal.DAY_OF_WEEK) gives correct output while cal.DAY_OF_WEEK is default and not modified?

问题描述

我想获取日期的日期.当我在cal.set(year，month-1，day)中设置日期时，设置日历字段 YEAR，MONTH，DAY_OF_MONTH，HOUR_OF_DAY和MINUTE 的值.保留其他字段的先前值.因此，当我执行cal.DAY_OF_WEEK时，我将获得已经存在的值.但是，cal.get(cal.DAY_OF_WEEK)如何给出正确的值?(如java docs中所述， public int get(int field)返回给定日历字段的值.)我不明白如何在此函数中传递整数值(作为日历)字段而未修改)给出正确的输出.

I want to get the day for the date. When i set the date in cal.set(year,month-1,day), Sets the values for the calendar fields YEAR, MONTH, DAY_OF_MONTH, HOUR_OF_DAY, and MINUTE. Previous values of other fields are retained. Thus, when I do cal.DAY_OF_WEEK, I will get the values which were existing already. But how does the cal.get(cal.DAY_OF_WEEK) gives the correct value? (As mentioned in java docs, public int get(int field) Returns the value of the given calendar field.) I don't understand how passing the integer value in this function (as calendar field that too un-modified) gives the correct output.

public static String findDay() {
 Calendar cal = Calendar.getInstance();
 cal.set(2019,1,15);
 System.out.println(cal.toString());
 System.out.println(cal.DAY_OF_WEEK); //7 which is unchanged by cal.set
 System.out.println(cal.get(cal.DAY_OF_WEEK)); //6
 return "";

}

代码输出:

java.util.GregorianCalendar[time=?,areFieldsSet=false,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Etc/UTC",offset=0,dstSavings=0,useDaylight=false,transi
tions=0,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2019,MONTH=1,WEEK_OF_YEAR=7,WEEK_OF_MONTH=3,DAY_OF_MONTH=15,DAY_OF_YEAR=46,DAY_OF_WEEK=6,DAY_OF_WEEK_IN
_MONTH=3,AM_PM=0,HOUR=11,HOUR_OF_DAY=11,MINUTE=38,SECOND=14,MILLISECOND=374,ZONE_OFFSET=0,DST_OFFSET=0]    
7
6

推荐答案

我们来剖析这些内容:

此:

System.out.println(cal.toString());

为您的Calendar对象打印 toString()，并显示其字段状态，并返回以下内容:

prints out the toString() for your Calendar object, the state of its fields, and returns this:

java.util.GregorianCalendar [time = ?, areFieldsSet = false，areAllFieldsSet = true，lenient = true，zone = sun.util.calendar.ZoneInfo [id ="Etc/UTC"，offset = 0，dstSavings = 0，useDaylight = false，transitions = 0，lastRule = null]，firstDayOfWeek = 1，minimumDaysInFirstWeek = 1，ERA = 1，YEAR = 2019，MONTH = 1，WEEK_OF_YEAR = 7，WEEK_OF_MONTH = 3，DAY_OF_MONTH = 15，DAY_OF_YEAR = 46，DAY_OF_WEEK = 6，DAY_OF_WEEK_IN_MONTH = 3，AM_PM = 0，HOUR = 11，HOUR_OF_DAY = 11，MINUTE = 38，SECOND = 14，MILLISECOND = 374，ZONE_OFFSET = 0，DST_OFFSET = 0]

java.util.GregorianCalendar[time=?,areFieldsSet=false,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Etc/UTC",offset=0,dstSavings=0,useDaylight=false,transi tions=0,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2019,MONTH=1,WEEK_OF_YEAR=7,WEEK_OF_MONTH=3,DAY_OF_MONTH=15,DAY_OF_YEAR=46,DAY_OF_WEEK=6,DAY_OF_WEEK_IN _MONTH=3,AM_PM=0,HOUR=11,HOUR_OF_DAY=11,MINUTE=38,SECOND=14,MILLISECOND=374,ZONE_OFFSET=0,DST_OFFSET=0]

是的，这里DAY_OF_WEEK是6

And yes, here DAY_OF_WEEK is 6

与此同时:

System.out.println(cal.DAY_OF_WEEK); //7 which is unchanged by cal.set

返回 Calendar.DAY_OF_WEEK 字段结果，该结果为常量，==等于7.

returns the Calendar.DAY_OF_WEEK field result, which is a constant and is == to 7.

最后这个:

System.out.println(cal.get(cal.DAY_OF_WEEK)); //6

返回6，这是您的Calendar对象保留的DAY_OF_WEEK字段.

returns 6, which is the DAY_OF_WEEK field held by your Calendar object.

将您的Calendar对象(简化)生动描述如下:

Picture your Calendar object (simplified) to look like this:

public abstract class Calendar {
    public static int DAY_OF_WEEK = 7;  // a constant that never changes

    // ..... many more fields

    // field name may be different
    private int dayOfWeek = 0; // or some other default value

    public void set(...) {
        // sets the dayOfWeek value
    }

    public int get(int ) {
        if (field == DAY_OF_WEEK) {
            return this.dayOfWeek;
        }

        // .... more code

    }

    public String toString() {
        return .... + "DAY_OF_WEEK:" + dayOfWeek + .....;
    }
}

现在有意义吗?您正在设置一个隐藏的dayOfWeek字段并获取

make sense now? There is a hidden dayOfWeek field that you're setting and getting the result of

所以这个:

System.out.println(cal.get(cal.DAY_OF_WEEK));

实际上返回 dayOfWeek 私有字段结果

actually returns the dayOfWeek private field result

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