Java 如何使用readObject（）从文件中读取复合类对象的数据？

如何使用readObject（）从文件中读取复合对象的数据？ 我的代码如下，我得到的输出为

version=4
purpose is DEMO
value=4
Exception in thread "main" java.lang.NullPointerException
        at Serializations.readObj(PS_Task3.java:39)
        at PS_Task3.main(PS_Task3.java:46)
Press any key to continue . . .

//PS_Task3.java //这是我试图执行的代码。当我在SerializationDemo中删除demo类型的变量时，得到了正确的输出。但当我尝试复合类时出现了NullPointerException。 请帮帮我，先谢谢你

import java.io.*;
class SerializationDemo implements Serializable //composite class
{
public int value = 4;
public demo d1;
public String purpose="DEMO";
public int getValue(){
    return value;
}
}
class demo{
    private int demoval;
    demo(){
        demoval=10;
    }
    public int getValue(){
        return demoval;
    }
}
class Serializations {
    void writeObj()throws IOException{   //writes into file
        FileOutputStream fos = new FileOutputStream("temp1.bin");
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        SerializationDemo sd = new SerializationDemo();
        oos.writeObject(sd);
        oos.flush();
        oos.close();
    }
    void readObj()throws IOException,ClassNotFoundException{ //reads from file
        FileInputStream fis = new FileInputStream("temp1.bin");
        ObjectInputStream oin = new ObjectInputStream(fis);
        SerializationDemo sd = (SerializationDemo) oin.readObject();
        System.out.println("version="+sd.value);
        System.out.println("purpose is "+sd.purpose);
        System.out.println("value="+sd.getValue());
        System.out.println("value="+sd.d1.getValue());
    }
}    
public class PS_Task3{
    public static void main(String args[]) throws IOException,ClassNotFoundException {
        Serializations s = new Serializations();
        s.writeObj();
        s.readObj();  //here i got the error
    }
}

---

请检查是否存在任何

null

void readObj()throws IOException,ClassNotFoundException{ //reads from file
       try{
        FileInputStream fis = new FileInputStream("temp1.bin");
        System.out.println("1->"+fis==null);
        ObjectInputStream oin = new ObjectInputStream(fis);
        System.out.println("2->"+oin==null);
        SerializationDemo sd = (SerializationDemo) oin.readObject();
        System.out.println("3->"+sd==null);
        System.out.println("version="+sd.value);
        System.out.println("purpose is "+sd.purpose);
        System.out.println("value="+sd.getValue());
        System.out.println("value="+sd.d1.getValue());
       }catch(Exception e)
         {
          System.out.println(e.toString);}
    }

---

请检查where是否实际为null，然后在那里执行操作。

要使序列化机制工作，所有类的本地成员变量必须是基元类型或可序列化对象

第一个问题是

SerializationDemo

有一个类型为

demo

的成员变量不可序列化

这将解决该问题：

class demo implements Serializable{
    private int demoval;
    demo(){
        demoval=10;
    }
    public int getValue(){
        return demoval;
    }
}

导致null异常的第二个问题是您从未设置

d1

的值。确保在序列化它之前设置了它，否则当您尝试反序列化它时，您将尝试从null创建演示类的实例。简单的解决方法是添加默认构造函数：

import java.io.*;
class SerializationDemo implements Serializable //composite class
{
    public SerializationDemo()
    {
        d1 = new demo();
    }
    public int value = 4;
    public demo d1;
    public String purpose="DEMO";
    public int getValue(){
    return value;
    }
}