Java IO流ArrayList对象写入文件有问题

Question

文件写入方法：public void writeObjectM(Object obj) throws Exception{  String file = "D:/1/data/Manager.dat";  ObjectOutputStream ooc = new ObjectOutputStream(    new FileOutputStream(file));    ooc.writeObject(obj);    ooc.flush();    ooc.close(); }//下面这种写入没有问题。Manager [] ma = {new Manager(1,2,"aaa"),new Manager(1,3,"bbb")};writeObjecM(ma);//因为程序中用到了ArrayList DB，在后面从ArrayList 保存对象到文件就不会了。。。Object m[] = readObjectM(); for (int i = 0; i < m.length ; i++){   Manager a = (Manager)m[i];   DB.add(a);  }//程序中修改了对象的属性，将对象再次保存到文件中就出错了。。。writeObjectM(DB);和for (int i = 0 ; i < DB.size(); i++)   {    Manager s = (Manager) DB.get(i);     writeObjectM(s);   }都出现问题，程序一开始的读入出错，学期大作业，真是不会改了，求大神帮忙。orz