import java.util.Calendar;
import java.text.SimpleDateFormat;
import java.util.Date;




public class DateUtil{


/**
* 获取指定日期间隔天数的起始值
* @param date
* @param pattern
* @param num
* @return
*/
public static String getDateStartNum(String date, String pattern, int num){
Calendar cal = Calendar.getInstance();
cal.setTime(stringToDate(date, pattern));
cal.add(Calendar.DATE, num);
return dateToString(cal.getTime(), pattern);
}


/**
* 获取当前日期的几天前或几天后日期
* @return
*/
public static String getCurrentBeforeNum(String pattern, int num){
Calendar cal = Calendar.getInstance();
cal.setTime(new Date());
cal.add(Calendar.DATE, num);
return dateToString(cal.getTime(), pattern);
}


/**
* 将日期字符串转化为日期
*
* @param date    日期字符串
* @param pattern 日期格式
* @return
*/
public static Date stringToDate(String date, String pattern) {
Date myDate = null;
if (date != null) {
try {
myDate = getDateFormat(pattern).parse(date);
} catch (Exception e) {
e.printStackTrace();
}
}
return myDate;
}
/**
* 将日期转化为日期字符串
*
* @param date    日期
* @param pattern 日期格式
* @return
*/
public static String dateToString(Date date, String pattern) {
String dateString = null;
if (date != null) {
try {
dateString = getDateFormat(pattern).format(date);
} catch (Exception e) {
e.printStackTrace();
}
}
return dateString;
}

/**
     * 获取SimpleDateFormat
     *
     * @param pattern 日期格式
     * @return
     */
    public static SimpleDateFormat getDateFormat(String pattern) {
        if (pattern != null && pattern.equals("")) {
            throw new RuntimeException("无效日期格式");
        }
        SimpleDateFormat dateFormat = new SimpleDateFormat(pattern);
        return dateFormat;
    }

/**
 * 获取间隔日期内的所有周
 * @param dBegin
 * @param dEnd
 * @return
 */
public static  List<Integer> findWeek(Date dBegin, Date dEnd) {
    List<Integer> week = new ArrayList<>();
    Calendar calendar = Calendar.getInstance();
    calendar.setFirstDayOfWeek(Calendar.MONDAY);
    calendar.setTime(dBegin);
    int startWeek = calendar.get(Calendar.WEEK_OF_YEAR);
    calendar.setTime(dEnd);
    int endWeek = calendar.get(Calendar.WEEK_OF_YEAR);
    for (; startWeek <= endWeek; startWeek ++) {
        week.add(startWeek);
    }
    return week;
}

public static void main(String[] args) {
// 调用 七天前的日期
System.out.print(DateUtil.getCurrentBeforeNum("yyyy-MM-dd", -7));
}




}