Get Monday

import java.util.Calendar;
import java.util.Date;

public class Main {
  public static void main(String[] argv) {
    System.out.println(getMonday(new Date()));
  }

  public static Date getMonday(Date today) {
    Calendar cal = Calendar.getInstance();

    cal.setTime(today);

    int dow = cal.get(Calendar.DAY_OF_WEEK);

    while (dow != Calendar.MONDAY) {
      int date = cal.get(Calendar.DATE);

      if (date == 1) {
        int month = cal.get(Calendar.MONTH);

        if (month == Calendar.JANUARY) {
          month = Calendar.DECEMBER;

          cal.set(Calendar.YEAR, cal.get(Calendar.YEAR) - 1);
        } else {
          month--;
        }

        cal.set(Calendar.MONTH, month);

        date = getMonthLastDate(month, cal.get(Calendar.YEAR));
      } else {
        date--;
      }

      cal.set(Calendar.DATE, date);

      dow = cal.get(Calendar.DAY_OF_WEEK);
    }

    return cal.getTime();
  }

  private static int getMonthLastDate(int month, int year) {
    switch (month) {
    case Calendar.JANUARY:
    case Calendar.MARCH:
    case Calendar.MAY:
    case Calendar.JULY:
    case Calendar.AUGUST:
    case Calendar.OCTOBER:
    case Calendar.DECEMBER:
      return 31;

    case Calendar.APRIL:
    case Calendar.JUNE:
    case Calendar.SEPTEMBER:
    case Calendar.NOVEMBER:
      return 30;

    default: // Calendar.FEBRUARY
      return year % 4 == 0 ? 29 : 28;
    }
  }
}