java 获取时间 本周本年本月第一天最后一天

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Calendar c = new GregorianCalendar();
      c.setFirstDayOfWeek(Calendar.MONDAY);
      c.setTime(new Date());
      c.set(Calendar.DAY_OF_WEEK, c.getFirstDayOfWeek()); // Monday
      System.out.println(c.getTime());
      c.set(Calendar.DAY_OF_WEEK, c.getFirstDayOfWeek() + 6); // Sunday 
 


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SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        Calendar c = Calendar.getInstance();
        
        c = Calendar.getInstance();
        c.set(Calendar.DAY_OF_WEEK, 1);//本周第一天，以星期日开始        
        System.out.println(sdf.format(c.getTime()));
        

        c = Calendar.getInstance();
        c.set(Calendar.DAY_OF_WEEK, 7);//本周最后一天
        System.out.println(sdf.format(c.getTime()));
        

        c = Calendar.getInstance();
        c.set(Calendar.DAY_OF_MONTH, 1);//本月第一天
        System.out.println(sdf.format(c.getTime()));
        
        c = Calendar.getInstance();