Q：对于一个格式化的日期字符串，读出这个日期，并要检测一下这个日期是否是有效的。
A：先parse()，再format()，比较一下两次的字符串是否相等。

import java.util.*;
import java.text.*;
public class 格式化日期 
{
    public static void main(String[] args) 
    {
        DateFormat format=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        String str="2017-11-25 06:40:99";
        try
        {
            //将格式化字符串转换为日期
            Date date=format.parse(str);
            System.out.println(date);
            //再次把时间转换为字符串，看转换后的字符串和之前被转换的字符串相等与否
            String temp=format.format(date);
            System.out.println(str.equals(temp));
            //得到时间：多少毫秒，从 1970-01-01 00:00:00 000 开始
            long time=date.getTime();
            System.out.println(time);
        }
        catch (Exception ex)
        {
        }
    }
}

Q：给出两个日期字符串，判断两个日期之间有几个星期四，有几个零点。
A：一种方法是将当前日期每次都加一，然后检测当前日期是否是星期四。另一种方法是计算两个日期之间有几天，就对应有相应个零点。

    public static int getThursday(String startStr,String endStr) throws Exception
    {
        DateFormat format=new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
        Date start=format.parse(startStr);
        Date end=format.parse(endStr);
        Calendar calendar=Calendar.getInstance();
        Date x=start;
        int count=0;
        calendar.setTime(x);
        while (x.compareTo(end)<0)
        {
            //System.out.println(x+"--->"+calendar.get(Calendar.DAY_OF_WEEK));
            if (calendar.get(Calendar.DAY_OF_WEEK)==Calendar.THURSDAY)
            {
                count++;
            }
            calendar.setTime(x);
            calendar.add(Calendar.DATE,1);
            x=calendar.getTime();
        }
        return count;
    }
    public static long getZeros(String startStr,String endStr) throws Exception
    {
        DateFormat format=new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
        Date start=format.parse(startStr);
        Date end=format.parse(endStr);
        long diff=end.getTime()-start.getTime();
        long zeros=diff/(24*60*60*1000);
        if (startStr.split(" ")[1].equals("0:0:0"))
        {
            zeros--;
        }
        return zeros;
    }

下面是Calendar类的一点源码

/**
     * Value of the {@link #DAY_OF_WEEK} field indicating
     * Sunday.
     */
    public final static int SUNDAY = 1;

    /**
     * Value of the {@link #DAY_OF_WEEK} field indicating
     * Monday.
     */
    public final static int MONDAY = 2;

    /**
     * Value of the {@link #DAY_OF_WEEK} field indicating
     * Tuesday.
     */
    public final static int TUESDAY = 3;

    /**
     * Value of the {@link #DAY_OF_WEEK} field indicating
     * Wednesday.
     */
    public final static int WEDNESDAY = 4;

    /**
     * Value of the {@link #DAY_OF_WEEK} field indicating
     * Thursday.
     */
    public final static int THURSDAY = 5;

    /**
     * Value of the {@link #DAY_OF_WEEK} field indicating
     * Friday.
     */
    public final static int FRIDAY = 6;

    /**
     * Value of the {@link #DAY_OF_WEEK} field indicating
     * Saturday.
     */
    public final static int SATURDAY = 7;