[Java]一个简易的生成流水号的类

无聊的时候写了一个简易的生成流水号的类生成的格式大致如下:

XXyyMMdd0001

import java.text.Format;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

/**
 * 
 * @author Phonnie
 *
 */
public class SerialNumber {
	
	private static final int MAX_VALUE=9999;
	private static final String FORMAT = "yyMMdd";
	private static final Format DF= new SimpleDateFormat(FORMAT);
	private static final byte[] lock = new byte[0];
	private String prefix = null;
	private Date date = null;
	private int number=1;
	private static Map<String, SerialNumber> map = new HashMap<String, SerialNumber>();
	
	private SerialNumber(String prefix,Date date){
		this.prefix = prefix;
		this.date = date;
	}
	
	public static SerialNumber newInstance(String prefix){
		Date date = new Date();
		return newInstance(prefix,date);
	} 
	
	public static SerialNumber newInstance(String prefix,Date date){
		SerialNumber o = null;
		synchronized (lock) {
			String key = getKey(prefix, date);
			if(map.containsKey(key)){
				o = map.get(key);
				int number = o.getNumber();
				if(number<MAX_VALUE){
					o.setNumber(number+1);
				}else {
					o.setNumber(1);
				}
				
			} else {
				 o = new SerialNumber(prefix,date);
				 map.put(key, o);
			}
		}
		return o;
	}
	
	
	
	private static String getKey(String prefix,Date date){
		return prefix+format(date);
	}

	private static String format(Date date){
		return DF.format(date);
	}
	
	public String toString(){
		return  prefix+ format(date) + String.format("%04d", number);
	}

	public void setNumber(int number) {
		this.number = number;
	}

	public int getNumber() {
		return number;
	}
}