날짜(Date, Calendar), 한주의 시작~끝 날짜, 요일, 마지막일

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

import com.ibm.icu.util.Calendar;


public class DateUtil {
	/**
	 * 월의 해당 주의 날짜 배열을 얻어온다.
	 * @param yyyymm
	 * @param weekSeq
	 * @return
	 */
	public static int[] getRangeDateOfWeek(String yyyymm, int weekSeq) {
		int rangeDateOfWeek [] = new int[7];
		
		int startDayOfWeek = dayOfWeek(yyyymm.substring(0, 4), yyyymm.substring(4, 6), "1");
		
		if( startDayOfWeek == 0 || weekSeq > 1 ){
			Calendar cal = converterDate(yyyymm+"01");
			int lastDateOfMonth = getLastDateOfMonth(new SimpleDateFormat("yyyyMM").format(cal.getTime()));
			
			int startDate = 1 + ((weekSeq-1)*7) - startDayOfWeek;
			for( int i=0; i<7; i++ ){
				if( startDate > lastDateOfMonth ){
					startDate = 1;
				}
				rangeDateOfWeek[i] = startDate++;
			}
		}else{
			Calendar cal = converterDate(yyyymm+"01");
			cal.add(Calendar.MONTH, -1);
			int lastDateOfBeforeMonth = getLastDateOfMonth(new SimpleDateFormat("yyyyMM").format(cal.getTime()));
			
			int startDate = (lastDateOfBeforeMonth + 1) - startDayOfWeek;
			for( int i=0; i<7; i++ ){
				if( startDate > lastDateOfBeforeMonth ){
					startDate = 1;
				}				
				rangeDateOfWeek[i] = startDate++;
			}
		}
		return rangeDateOfWeek;
	}
	/**
	 * 특정날짜의  요일의 숫자를 리턴
	 * 0:일요일 ~ 6:토요일
	 * @return
	 */
	public static int dayOfWeek(String sYear, String sMonth, String sDay) {		
		
		int iYear = Integer.parseInt(sYear);
		int iMonth = Integer.parseInt(sMonth) - 1;
		int isDay = Integer.parseInt(sDay);
		
		GregorianCalendar gc = new GregorianCalendar(iYear, iMonth, isDay); 

		return gc.get(gc.DAY_OF_WEEK) - 1;				
	}
	
	/**
	 * String 형식의 날자를 Calendar 로 변환 해준다.
	 * 
	 * @param yyyymmdd
	 * @return
	 */
	public static Calendar converterDate(String yyyymmdd) {
		Calendar cal = Calendar.getInstance(); // 양력 달력
		if (yyyymmdd == null)
			return cal;

		String date = yyyymmdd.trim();
		if (date.length() != 8) {
			if (date.length() == 4)
				date = date + "0101";
			else if (date.length() == 6)
				date = date + "01";
			else if (date.length() > 8)
				date = date.substring(0, 8);
			else
				return cal;
		}

		cal.set(Calendar.YEAR, Integer.parseInt(date.substring(0, 4)));
		cal.set(Calendar.MONTH, Integer.parseInt(date.substring(4, 6)) - 1);
		cal.set(Calendar.DAY_OF_MONTH, Integer.parseInt(date.substring(6)));

		return cal;
	}
	
	/**
	 * 해당 월의 마지막일을 구한다.
	 * @return
	 */
	public static int getLastDateOfMonth() {
		return getLastDateOfMonth(new Date());
	}
	public static int getLastDateOfMonth(Date date) {
		return getLastDateOfMonth(new SimpleDateFormat("yyyyMM").format(date));
	}
	public static int getLastDateOfMonth(String yyyymm) {
		int year = Integer.parseInt(yyyymm.substring(0, 4));
		int month = Integer.parseInt(yyyymm.substring(4, 6)) - 1;
		
		Calendar destDate = Calendar.getInstance();
		destDate.set(year, month, 1);
		
		return destDate.getActualMaximum(Calendar.DATE);
	}
}