一个简易的流水账号实现方式(java)

private static Hashtable<String, Integer> seq = new Hashtable<String, Integer>();
public synchronized static String getOrderSeq(){
String rtv = "0000";
String datetimeKey=new SimpleDateFormat("yyyyMMdd").format(new Date());
if ( seq.keySet().size()==0){
seq.put(datetimeKey, 0);
} else {
if ( seq.get(datetimeKey) == null){
seq.clear();
seq.put(datetimeKey, 0);
} else {
int order =seq.get(datetimeKey)+1;
String orderStr = ""+order;
int len= rtv.length();
int sub=orderStr.length();
if ( len-sub>=0){
rtv = rtv.substring(0,len-sub);
rtv = rtv+ orderStr;
}
else
rtv = orderStr;
seq.put(datetimeKey,order);
}
}
rtv = datetimeKey +rtv;
return rtv;
}