code: public static String yyyy_MM_dd(String strDate) {
          SimpleDateFormat formatter = new SimpleDateFormat(dateFormat);
          ParsePosition pos = new ParsePosition(0);
          Date strtodate = formatter.parse(strDate, pos);
          SimpleDateFormat formatter2 = new SimpleDateFormat(dateFormat);
          return formatter2.format(strtodate);
      }
docs: 将以yyyy-MM-dd开头的日期字符串格式化为yyyy-MM-dd短日期格式
  
  @param strDate eg. 2015-10-11 11:11:11
  @return eg. 2015-10-11 12:12:12  to  2015-10-11