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


import org.junit.Test;


public class TestDateUtil {
//1、获取当月第一天
@Test
public void testForDate(){
//规定返回日期格式
SimpleDateFormat sf=new SimpleDateFormat("yyyy-MM-dd");
Calendar calendar=Calendar.getInstance();
Date theDate=calendar.getTime();
GregorianCalendar gcLast=(GregorianCalendar)Calendar.getInstance();
gcLast.setTime(theDate);
//设置为第一天
gcLast.set(Calendar.DAY_OF_MONTH, 1);
String day_first=sf.format(gcLast.getTime());
//打印本月第一天
System.out.println(day_first);
}
//2、获取当月最后一天
@Test
public void testForDatelast(){
//获取Calendar
Calendar calendar=Calendar.getInstance();
//设置日期为本月最大日期
calendar.set(Calendar.DATE, calendar.getActualMaximum(calendar.DATE));
//设置日期格式
SimpleDateFormat sf=new SimpleDateFormat("yyyy-MM-dd");
String ss=sf.format(calendar.getTime());
System.out.println(ss+" 23:59:59");
}
//3、非常简单和实用的获取本月第一天和最后一天
@Test
public void testt(){
SimpleDateFormat format=new SimpleDateFormat("yyyy-MM-dd");
Calendar c = Calendar.getInstance();    
       c.add(Calendar.MONTH, 0);
       c.set(Calendar.DAY_OF_MONTH,1);//设置为1号,当前日期既为本月第一天 
       String first = format.format(c.getTime());
       System.out.println("===============本月first day:"+first);
       
       //获取当前月最后一天
       Calendar ca = Calendar.getInstance();    
       ca.set(Calendar.DAY_OF_MONTH, ca.getActualMaximum(Calendar.DAY_OF_MONTH));  
       String last = format.format(ca.getTime());
       System.out.println("===============本月last day:"+last);
}
//4、获取上个月的第一天
@Test
public void getBeforeFirstMonthdate()throws Exception{
SimpleDateFormat format=new SimpleDateFormat("yyyy-MM-dd");
Calendar calendar=Calendar.getInstance();
calendar.add(Calendar.MONTH, -1);
calendar.set(Calendar.DAY_OF_MONTH, 1);
System.out.println("上个月第一天："+format.format(calendar.getTime()));
}
//5、获取上个月的最后一天
@Test
public void getBeforeLastMonthdate()throws Exception{
SimpleDateFormat sf=new SimpleDateFormat("yyyy-MM-dd");
Calendar calendar=Calendar.getInstance();
int month=calendar.get(Calendar.MONTH);
calendar.set(Calendar.MONTH, month-1);
calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMaximum(Calendar.DAY_OF_MONTH));
System.out.println("上个月最后一天："+sf.format(calendar.getTime()));
}

6、获取当前日期的前一周或者前一个月时间

 public static String getFirstDate(){
SimpleDateFormat  sdf = new SimpleDateFormat("yyyyMMdd");
Calendar cl = Calendar.getInstance();
// cl.setTime(dateNow);
// cl.add(Calendar.DAY_OF_YEAR, -1);
//一天
// cl.add(Calendar.WEEK_OF_YEAR, -1);
//一周
cl.add(Calendar.MONTH, -1);
//从现在算，之前一个月,如果是2个月，那么-1-----》改为-2
Date dateFrom = cl.getTime();
return sdf.format(dateFrom);
 }

 @Test
 public void testStartDate(){
 System.out.println("当前日期往前推一个月是："+getFirstDate());

 //如果当前日期是2015.11.08,那么打印日期是:20151008
 }
}