DateUtil

public class DateUtil {
/**
* 规范化日期，规范成yyyy-MM-dd HH:mm:ss
* @param timestamp
* @return
*/
public static String timestamp2Datetime(long timestamp){
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
return dateFormat.format(new Date(timestamp * 1000));
}
/**
* 规范化日期，规范成yyyy-MM-dd
* @param timestamp
* @return
*/
public static String timestamp2Date(long timestamp){
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
return dateFormat.format(new Date(timestamp * 1000));
}
/**
* 规范化日期，规范成yyyy-MM-dd HH:mm:ss
* @param date
* @return
*/
public static String Date2DateString(Date date){
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
return dateFormat.format(date);
}
/**
* 规范化日期，规范成yyyy-MM-dd
* @param date
* @return
*/
public static String Date2String(Date date){
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
return dateFormat.format(date);
}
public static int Stringdate2Intdate(String date){
String [] str = date.split("-");
StringBuffer sb = new StringBuffer();
for(int i=0;i<str.length;i++){
sb.append(str[i]);
}

return Integer.valueOf(sb.toString());
}
/**
* yyyyMMdd转成long型数据
* @param datestr
* @return
*/
public static long date3Timestamp(String datestr){
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
Date date = new Date();
try {
date = dateFormat.parse(datestr);
} catch (ParseException e) {
e.printStackTrace();
}
long timestamp = date.getTime()/1000;
return timestamp;
}

/**
*获得当前时间
* @return
*/
public static String getCurrentDatetime(){
Date date=new Date();
SimpleDateFormat df=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
return df.format(date);
}

/**
* 获得当前日期
* @return
*/
public static String getCurrentFormatDate2(){
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
Date date = new Date();
return dateFormat.format(date);
}

/**
* 日期相加，并按照要求日期格式输出 ,示例：20120305
* @param date 被操作日期
* @param days 被加天数
* @return 标准化操作后日期
*/
public static String addDate(java.util.Date date,int days){
java.util.Calendar calendar = java.util.Calendar.getInstance();
java.text.SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyyMMdd");
calendar.setTimeInMillis(getMillis(date) +  ((long)  days)  *  24  *  3600  *  1000);
return simpleDateFormat.format(calendar.getTime());
}

/**
* 日期相减，并按照要求日期格式输出 ,示例：20120305
* @param date 被操作日期
* @param days 被加天数
* @return 标准化操作后日期
*/
public static String diffDate(java.util.Date date,int days){
java.util.Calendar calendar = java.util.Calendar.getInstance();
java.text.SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyyMMdd");
calendar.setTimeInMillis(getMillis(date) - ((long)  days)  *  24  *  3600  *  1000);
return simpleDateFormat.format(calendar.getTime());
}
/**
* 返回毫秒数
* @param date 日期
* @return 返回毫秒
*/
public static long getMillis(java.util.Date date){
java.util.Calendar calendar = java.util.Calendar.getInstance();
calendar.setTime(date);
return calendar.getTimeInMillis();
}
/**
* 获取上个月第一天
*/
public static String getFirstDayofLastMonth(int day){
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
Calendar c = Calendar.getInstance();
c.add(Calendar.MONTH, -day);
Date thedate = c.getTime();

Calendar l = Calendar.getInstance();
l.setTime(thedate);
l.set(Calendar.DAY_OF_MONTH, 1);
String firstday = dateFormat.format(l.getTime());
return firstday;
}
/**
* 获取本月第一天
* @return
*/
public static String getFirstDayOfThisMonth(){
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
Calendar c = Calendar.getInstance();
c.set(Calendar.DAY_OF_MONTH, 1);
String firstday = dateFormat.format(c.getTime());
return firstday;
}

public static String getYesterday(){
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
Calendar c = Calendar.getInstance();
c.add(Calendar.DAY_OF_MONTH, -1);
String firstday = dateFormat.format(c.getTime());
return firstday;
}

public static String getCurrentYearMonth(String date){
SimpleDateFormat df=new SimpleDateFormat("yyyyMM");
SimpleDateFormat df1=new SimpleDateFormat("yyyy-MM-dd");
Date dates = new Date();
try {
dates =df1.parse(date);

} catch (ParseException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return df.format(dates);
}

public static void main(String []args){
System.out.println(getFirstDayOfThisMonth());
System.out.println(getFirstDayofLastMonth(3));
System.out.println(getYesterday());
System.out.println(getCurrentFormatDate2());
System.out.println(getCurrentYearMonth("2014-09-01"));
System.out.println(DateUtil.getFirstDayofLastMonth(2));
}
}