Java Practice: Object Serialization

今天的 Java Practice 如下, 請服用:

1. Coper 類別的功用為何?
2. Coper:: <T> copyInstance(T obj) 方法使用了 Java Generic 的技巧，請解釋這裡採用型別參數的好處?
3. main 方法中印出的兩個 User 物件，內容是否相同? 若有差異，原因為何?

abstract class Copier {
    @SuppressWarnings("unchecked")
    public static <T> copyInstance(T obj) throws IOException,
            ClassNotFoundException {
        ByteArrayOutputStream outBytes = new ByteArrayOutputStream();
        ObjectOutputStream outStream = new ObjectOutputStream(outBytes);
        outStream.writeObject(obj);
        outStream.close();
        ByteArrayInputStream inBytes = new ByteArrayInputStream(outBytes
                .toByteArray());
        ObjectInputStream inStream = new ObjectInputStream(inBytes);
        T copied = (T) inStream.readObject();
        inStream.close();
        return copied;
    }
}

class User implements Serializable {
    private static final long serialVersionUID = 1L;
    private Date date = new Date();
    private String username;

    private transient String password;

    public User(String name, String pwd) {
        username = name;
        password = pwd;
    }

    public String toString() {
        String pwd = (password == null) ? "(n/a)" : password;
        return "logon info: \n   username: " + username 
            + "\n   date: " + date
            + "\n   password: " + pwd;
    }
}

public class CopierTest {
    /**
     * @param args
     * @throws Exception
     * @throws IOException
     */
    public static void main(String[] args) throws Exception {
        User jack = new User("Jack", "magic");
        System.out.println(jack);
        User copyOfJack = Copier.copyInstance(jack);
        System.out.println(copyOfJack);
    }
}