Android中的HashMap反序列化问题

Question

我使用以下代码在我的PC应用程序上序列化HashMap:

private void serialize(HashMap<Integer, Integer> map2write, String name_ser)
{// serializes fphlist into .ser file called name_ser
    FileOutputStream fileOut = null;
    try {
        fileOut = new FileOutputStream(project_dir + "/" + name_ser + ".ser");
    } catch (FileNotFoundException ex) {
        Logger.getLogger(AdminConsoleUI.class.getName()).log(Level.SEVERE, null, ex);
    }
    ObjectOutputStream out;
    try {
        out = new ObjectOutputStream(fileOut);
        out.writeObject(map2write);
        out.reset();
        out.flush();
        out.close();
        fileOut.close();

    } catch (IOException ex) {
        Logger.getLogger(AdminConsoleUI.class.getName()).log(Level.SEVERE, null, ex);
    }
}

然后我使用以下代码在我的Android应用程序中对其进行反序列化:

private HashMap<Integer,Integer> deserialize_Map(String fn)
{// deserializes fn into HashMap

    HashMap<Integer,Integer> hm = new HashMap<Integer,Integer>();
    try
    {
        FileInputStream fileIn = new FileInputStream(project_dir + "/" + fn + ".ser");
        ObjectInputStream in = new ObjectInputStream(fileIn);
        hm = (HashMap<Integer,Integer>) in.readObject();
        in.close();
        fileIn.close();

   }catch(IOException i)
   {
       Log.e("MYAPP", "exception", i);
       return null;
   }catch(ClassNotFoundException c)
   {
       Log.e("MYAPP", "exception", c);
       return null;
   }catch(ClassCastException ex)
   {
       Log.e("MYAPP", "exception", ex);
       return null;
   }

    return hm;
}

最后,我面临两个问题.

1)反序列化需要很长时间.它包含大约数千个键,这是正常的吗？是否有更有效的序列化方法来解决这个问题？

2)反序列化后,我得到的哈希映射占用了它最初在VM上占用的大小的两倍,当我在调试器中检查它时,它在最初应该包含的键值之间有很多空条目.但是,它们不是null键,而只是null,我无法查看它们内部的内容.我在Eclipse中调试.为什么会这样？