java获取当月天数，指定年月的天数，指定日期获取对应星期 .

 package huolongluo.family.util;

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

 /**
  * Created by 火龙裸 on 2018/7/13.
  */

 public class Number_Of_Days
 {
     /**
      * 获取当月的 天数
      */
     public static int getCurrentMonthDay()
     {
         Calendar a = Calendar.getInstance();
         a.set(Calendar.DATE, 1);
         a.roll(Calendar.DATE, -1);
         int maxDate = a.get(Calendar.DATE);
         return maxDate;
     }

     /**
      * 根据 年、月 获取对应的月份 的 天数
      */
     public static int getDaysByYearMonth(int year, int month)
     {
         Calendar a = Calendar.getInstance();
         a.set(Calendar.YEAR, year);
         a.set(Calendar.MONTH, month - 1);
         a.set(Calendar.DATE, 1);
         a.roll(Calendar.DATE, -1);
         int maxDate = a.get(Calendar.DATE);
         return maxDate;
     }

     /**
      * 根据日期 找到对应日期的 星期几
      *
      * @param date 比如传参：2018-07-13 将返回“周五”
      */
     public static String getDayOfWeekByDate(String date)
     {
         String dayOfweek = "-1";
         try
         {
             SimpleDateFormat myFormatter = new SimpleDateFormat("yyyy-MM-dd");
             Date myDate = myFormatter.parse(date);
             SimpleDateFormat formatter = new SimpleDateFormat("E");
             String str = formatter.format(myDate);
             dayOfweek = str;
         }
         catch (Exception e)
         {
             System.out.println("错误!");
         }
         return dayOfweek;
     }
 }

这里添加另一个获取具体星期几的获取方法：

/**
 * 根据当前日期获得是星期几
 * time=yyyy-MM-dd
 * @return
 */
public static String getWeek(String time) {
    String Week = "";
    SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
    Calendar c = Calendar.getInstance();
    try {
        c.setTime(format.parse(time));
    } catch (ParseException e) {
        e.printStackTrace();
    }

    int wek=c.get(Calendar.DAY_OF_WEEK);

    if (wek == 1) {
        Week += "星期日";
    }
    if (wek == 2) {
        Week += "星期一";
    }
    if (wek == 3) {
        Week += "星期二";
    }
    if (wek == 4) {
        Week += "星期三";
    }
    if (wek == 5) {
        Week += "星期四";
    }
    if (wek == 6) {
        Week += "星期五";
    }
    if (wek == 7) {
        Week += "星期六";
    }
    return Week;
}

补充一条，获取系统当前时间：

final DateFormat df = DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM);
String comment = "Added on " + df.format(new Date());
Log.e("当前时间为" + comment);

打印出来的格式为：xxxx年xx月xx日 xx:xx:xx