import *;

public class TestObject {
    public static void main(String[] args){
        TestSer ts = new TestSer();
        TestExt te = new TestExt();
        try{
            FileOutputStream fos = new FileOutputStream("D:/JAVA/testStream/object.txt");
            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(te);
            oos.flush();
            oos.close();
            
            FileInputStream fis = new FileInputStream("D:/JAVA/testStream/object.txt");
            ObjectInputStream ois = new ObjectInputStream(fis);
            te = (TestExt)ois.readObject(); //**********************
            ois.close();
            
        }    catch(FileNotFoundException e){
            System.out.println("FileNotFoundException");
            e.printStackTrace();
            System.exit(-1);
            
        }    catch(IOException e){
            System.out.println("IOException");
            e.printStackTrace();
            System.exit(-1);
            
        }    catch(ClassNotFoundException e){
            System.out.println("ClassNotFoundException");
            e.printStackTrace();
            System.exit(-1);
        }
    }
}

//继承接口测试类
class TestExt implements Externalizable {
    String s = "hello";
   
    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        s = (String)in.readObject();
    }
   
    public void writeExternal(ObjectOutput out) throws IOException {
        out.writeObject(s);
    }
}

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注意上面程序用注释***********标记的地方，那里抛出了
想了很久也不知道为什么。求大神们解答。。。
谢谢

第一句忽略，我删除没有注释的时候删除漏了的！