IO 入门级问题，谢谢

[oicap1]

为什么我把 sd=(Student)ois.readObject(); 这段改成
Student sd2=(Student)ois.readObject();
然后下面打印的地方改成 sd2.age 就会报错。

import java.io.*;
class Serialization 
{
	public static void main(String[] args) throws IOException,ClassNotFoundException
	{
		Student sd=new Student(19,"dintdding",20,"computer");
		FileOutputStream fos=new FileOutputStream("Myfile.txt");
		ObjectOutputStream oos=new ObjectOutputStream(fos);
		try
		{
			oos.writeObject(sd);
			oos.close();
		}
		catch(Exception e)
		{
			System.out.println(e.toString());	
		}
		sd=null;
		FileInputStream fis=new FileInputStream("Myfile.txt");
		ObjectInputStream ois=new ObjectInputStream(fis);
		try
		{
			sd=(Student)ois.readObject();
			fis.close();
		}
		catch(Exception e)
		{
			System.out.println(e.toString());
		}
		System.out.println("id = "+sd.id);
		System.out.println("name = "+sd.name);
		System.out.println("age = "+sd.age);
		System.out.println("department = "+sd.department);
	}
}

class Student implements Serializable
{
	int id;
	String name;
	int age;
	String department;
	public Student(int id,String name,int age,String department)
	{
		this.id=id;
		this.name=name;
		this.age=age;
		this.department=department;	
	}
	public Student()
	{
	}
}

[shuwei003]

因为你sd2是在try中定义的，如果出现异常，下面不知道sd2是什么东东。所以，如果用sd2，应该在try前面定义Student sd2

[maen1987430]

局部变量问题。

[fengnenglu]

你有一行把sd=null了，当然会报空指针异常啦！

[maen1987430]

局部变量问题。

[fengnenglu]

package test;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class Test3
{
public static void main(String[] args) throws IOException,ClassNotFoundException
{
Student sd=new Student(19,"dintdding",20,"computer");
Student sd2 = null;
FileOutputStream fos=new FileOutputStream("Myfile.txt");
ObjectOutputStream oos=new ObjectOutputStream(fos);
try
{
oos.writeObject(sd);
oos.close();
}
catch(Exception e)
{
System.out.println(e.toString());
}
sd=null; FileInputStream fis=new FileInputStream("Myfile.txt");
ObjectInputStream ois=new ObjectInputStream(fis);
try
{
sd2=(Student)ois.readObject();
fis.close();
}
catch(Exception e)
{
System.out.println(e.toString());
}
System.out.println("id = "+sd2.id);
System.out.println("name = "+sd2.name);
System.out.println("age = "+sd2.age);
System.out.println("department = "+sd2.department);
}
}

class Student implements Serializable
{
int id;
String name;
int age;
String department;
public Student(int id,String name,int age,String department)
{
this.id=id;
this.name=name;
this.age=age;
this.department=department;
}
public Student()
{
}
}

[oicap1]

多谢LS各位，确实是局部变量的问题。新手，前面没学好。
没意识到try代码块里的变量的作用域。
顺便问下，是不是一对大括号内定义的变量，作用域都在这对大括号内?

[brightyq]

嗯，LZ程序是try catch问题，不是IO问题。

[niuniu20008]

[Quote=引用 6 楼 oicap1 的回复:]
多谢LS各位，确实是局部变量的问题。新手，前面没学好。
没意识到try代码块里的变量的作用域。
顺便问下，是不是一对大括号内定义的变量，作用域都在这对大括号内?
[/Quote]
是的。