java 日期获取，每月一号，每周一

java 日期获取，每月一号，每周一 

package info.lumanman.h5.util;

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

public class DateUtil {

    public static void main(String[] args) {
       SimpleDateFormat sdf= new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
        //addDay(sdf,10);
        //getWeekDay(sdf);
       getMonth(sdf);

   }
    /**
    * 增加天数
    * @param sdf
    */
    public static void addDay(SimpleDateFormat sdf,int days){

        //获取Calendar实例，java封装的表现，private类Calendar的构造函数，通过静态方法创建对象
       Calendar calendar=Calendar.getInstance();
        //获取当前时间并格式化
       System. out.println("当前时间: "+sdf.format(calendar.getTime()));

        //给当前时间加上天数days并输出，这里的第一个参数用以下三个都可以
       Calendar calendar1=Calendar.getInstance();
       calendar1. add(Calendar.DAY_OF_YEAR, days);
       System. out.println("加了"+days+"天之后的时间: "+sdf.format(calendar1.getTime()));

       Calendar calendar2=Calendar.getInstance();
       calendar2. add(Calendar.DAY_OF_MONTH, days);
       System. out.println("加了"+days+"之后的时间: "+sdf.format(calendar2.getTime()));

       Calendar calendar3=Calendar.getInstance();
       calendar3. add(Calendar.DAY_OF_WEEK, days);
       System. out.println("加了"+days+"之后的时间: "+sdf.format(calendar3.getTime()));
   }
    /**
    * 获取当前星期、前一星期、后一星期的第一天和最后一天
    * @param sdf
    */
    public static void getWeekDay(SimpleDateFormat sdf){
        //获取Calendar实例，java封装的表现，private类Calendar的构造函数，通过静态方法创建对象
       Calendar calendar=Calendar.getInstance();
        //获取当前时间并格式化
       System. out.println("当前时间: "+sdf.format(calendar.getTime()));

        //本周一
       Calendar calendar1=Calendar.getInstance();
       calendar1. set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
       System. out.println("本周一: "+sdf.format(calendar1.getTime()));
        //本周日
       Calendar calendar2=Calendar.getInstance();
        //外国的星期天和我们的不在一周(外国是星期天到星期六为一个星期)
       calendar2. add(Calendar.DAY_OF_WEEK, 7);
       calendar2. set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);
       System. out.println("本周日: "+sdf.format(calendar2.getTime()));

        //下周一
       Calendar calendar3=Calendar.getInstance();
       calendar3. add(Calendar.DAY_OF_MONTH, 7);
       calendar3. set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
       System. out.println("下周一: "+sdf.format(calendar3.getTime()));
        //下周日
       Calendar calendar4=Calendar.getInstance();
       calendar4. add(Calendar.DAY_OF_MONTH, 14);
       calendar4. set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);
       System. out.println("下周日: "+sdf.format(calendar4.getTime()));

        //上周一
       Calendar calendar5=Calendar.getInstance();
       calendar5. add(Calendar.DAY_OF_YEAR, -7);
       calendar5. set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
       System. out.println("上周一: "+sdf.format(calendar5.getTime()));
        //上周日
       Calendar calendar6=Calendar.getInstance();
       calendar6. set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);
       System. out.println("上周日: "+sdf.format(calendar6.getTime()));
   }
    /**
    * 获取当前月、前一月、后一月的第一天和最后一天
    * @param sdf
    */
    public static void getMonth(SimpleDateFormat sdf){
        //定义当前月的总天数，比如30,31,28,29
        int maxCurrentMonthDay=0;
       Calendar calendar=Calendar.getInstance();
       System. out.println("当前时间: "+sdf.format(calendar.getTime()));

        //当月一号
       Calendar calendar1=Calendar.getInstance();
       calendar1. set(Calendar.DAY_OF_MONTH, 1);
       System. out.println("本月第一天: "+sdf.format(calendar1.getTime()));
        //当月最后一天
       Calendar calendar2=Calendar.getInstance();
       maxCurrentMonthDay=calendar2.getActualMaximum(Calendar.DAY_OF_MONTH);
       calendar2. set(Calendar.DAY_OF_MONTH, maxCurrentMonthDay);
       System. out.println("本月最后一天: "+sdf.format(calendar2.getTime()));

        //下个月一号
       Calendar calendar3=Calendar.getInstance();
       maxCurrentMonthDay=calendar3.getActualMaximum(Calendar.DAY_OF_MONTH);
       calendar3. add(Calendar.DAY_OF_MONTH, maxCurrentMonthDay);
       calendar3. set(Calendar.DAY_OF_MONTH, 1);
       System. out.println("下月第一天: "+sdf.format(calendar3.getTime()));
        //下个月最后一天
       Calendar calendar4=Calendar.getInstance();
       maxCurrentMonthDay=calendar4.getActualMaximum(Calendar.DAY_OF_MONTH);
       calendar4. add(Calendar.DAY_OF_MONTH, maxCurrentMonthDay);
        //第一个maxCurrentMonthDay获取的是当月的天数，第二个maxCurrentMonthDay获取的是下个月的天数
       maxCurrentMonthDay=calendar4.getActualMaximum(Calendar.DAY_OF_MONTH);
       calendar4. set(Calendar.DAY_OF_MONTH, maxCurrentMonthDay);
       System. out.println("下月第一天: "+sdf.format(calendar4.getTime()));

        //上个月一号
       Calendar calendar5=Calendar.getInstance();
       maxCurrentMonthDay=calendar5.getActualMaximum(Calendar.DAY_OF_MONTH);
       calendar5. add(Calendar.DAY_OF_MONTH, -maxCurrentMonthDay);
       calendar5. set(Calendar.DAY_OF_MONTH, 1);
       System. out.println("上月第一天: "+sdf.format(calendar5.getTime()));
        //上个月最后一天
       Calendar calendar6=Calendar.getInstance();
       maxCurrentMonthDay=calendar6.getActualMaximum(Calendar.DAY_OF_MONTH);
       calendar6. add(Calendar.DAY_OF_MONTH, -maxCurrentMonthDay);
        //第一个maxCurrentMonthDay获取的是当月的天数，第二个maxCurrentMonthDay获取的是上个月的天数
       maxCurrentMonthDay=calendar6.getActualMaximum(Calendar.DAY_OF_MONTH);
       calendar6. set(Calendar.DAY_OF_MONTH, maxCurrentMonthDay);
       System. out.println("上月第一天: "+sdf.format(calendar6.getTime()));

   }