JAVA中Calendar与Date类型互转

 Java 语言的Date(日期)，Calendar(日历)，DateFormat(日期格式)组成了Java标准的一个基本但是非常重要的部分。

Date与String互转详情：https://blog.csdn.net/fz13768884254/article/details/82415432

(1) Calendar转化为Date  
    Calendar cal=Calendar.getInstance();  
    Date date=cal.getTime();  
      
    (2) Date转化为Calendar  
    Date date=new Date();  
    Calendar cal=Calendar.getInstance();  
    cal.setTime(date);  

Calendar中时间转换方法： 

 /**
     * Returns a <code>Date</code> object representing this
     * <code>Calendar</code>'s time value (millisecond offset from the <a
     * href="#Epoch">Epoch</a>").
     *
     * @return a <code>Date</code> representing the time value.
     * @see #setTime(Date)
     * @see #getTimeInMillis()
     */
    public final Date getTime() {
        return new Date(getTimeInMillis());
    }

    /**
     * Sets this Calendar's time with the given <code>Date</code>.
     * <p>
     * Note: Calling <code>setTime()</code> with
     * <code>Date(Long.MAX_VALUE)</code> or <code>Date(Long.MIN_VALUE)</code>
     * may yield incorrect field values from <code>get()</code>.
     *
     * @param date the given Date.
     * @see #getTime()
     * @see #setTimeInMillis(long)
     */
    public final void setTime(Date date) {
        setTimeInMillis(date.getTime());
    }

    /**
     * Returns this Calendar's time value in milliseconds.
     *
     * @return the current time as UTC milliseconds from the epoch.
     * @see #getTime()
     * @see #setTimeInMillis(long)
     */
    public long getTimeInMillis() {
        if (!isTimeSet) {
            updateTime();
        }
        return time;
    }

    /**
     * Sets this Calendar's current time from the given long value.
     *
     * @param millis the new time in UTC milliseconds from the epoch.
     * @see #setTime(Date)
     * @see #getTimeInMillis()
     */
    public void setTimeInMillis(long millis) {
        // If we don't need to recalculate the calendar field values,
        // do nothing.
        if (time == millis && isTimeSet && areFieldsSet && areAllFieldsSet
            && (zone instanceof ZoneInfo) && !((ZoneInfo)zone).isDirty()) {
            return;
        }
        time = millis;
        isTimeSet = true;
        areFieldsSet = false;
        computeFields();
        areAllFieldsSet = areFieldsSet = true;
    }

public void getTimeByCalendar(){
        Calendar cal = Calendar.getInstance();
        int year = cal.get(Calendar.YEAR);//获取年份
        int month=cal.get(Calendar.MONTH);//获取月份
        int day=cal.get(Calendar.DATE);//获取日
        int hour=cal.get(Calendar.HOUR);//小时
        int minute=cal.get(Calendar.MINUTE);//分           
        int second=cal.get(Calendar.SECOND);//秒
        int WeekOfYear = cal.get(Calendar.DAY_OF_WEEK);//一周的第几天
        System.out.println("现在的时间是：公元"+year+"年"+month+"月"+day+"日      "+hour+"时"+minute+"分"+second+"秒       星期"+WeekOfYear);
    }