NullPointerException when trying to write object instance to ObjectOutputStream

问题

I'm getting

Exception in thread "main" java.lang.NullPointerException
    at java.io.FileOutputStream.<init>(FileOutputStream.java:201)
    at java.io.FileOutputStream.<init>(FileOutputStream.java:99)
    at lib.Entry.serialize(Entry.java:17)
    at main.Main.main(Main.java:8)

Where Entry.java:17 is stream.writeObject(this); (see below)

Entry.java

package lib;
import java.io.*;

public class Entry { // Superclass.

    String filename; // Set below.
    String name; // Set by the subclass.

    public void main() {
        this.filename = this.name + ".ser";
        serialize();
    }

    public void serialize() {           
        try {
            FileOutputStream file = new FileOutputStream(this.filename);
            ObjectOutputStream stream = new ObjectOutputStream(file);
            stream.writeObject(this);
            stream.close();
            file.close();
            System.out.println("Serialized.");
        } catch(IOException e) {
            e.printStackTrace();
        }
    }

}

Place.java

package lib;

public class Place extends lib.Entry { // A subclass.

    public String name;

    public Place(String name) {
        this.name = name;
    }

}

Main.java

package main;
import lib.Place;

public abstract class Main {

    public static void main(String[] args) {
        Place room = new Place("room");
        room.serialize();
    }

}

Why am I getting a NullPointerException when using this? I'm trying to write the current object instance to the ObjectOutputStream. I'm new to Java and I have no idea how to proceed. In Python I'd use something like stream.writeObject(self) so by the same line of thought I used this in Java. I tried using stream.writeObject(Object this);, but it didn't work. I also tried

Object p = this;
stream.writeObject(p);

Which I guess is the same thing. It also didn't work. The idea is to have more classes (other than Place) extending Entry, allowing them to be serialized using the Entry.serialize() method.

回答1:

String name; // Set by the subclass.

That's the problem. Since you have re-defined the name field in subclass, it will no longer set the field in the super class using this.name. The name field in Place class shadows the field declared in Entry class.

Remove the declaration of name from Place class.

Then replace the main() method in Entry class with a parameterized constructor. I don't know why you had it in the first place. You aren't even calling it.

public Entry(String name) {
    this.name = name;
    this.filename = this.name + ".ser";

    // Don't call it here. Your object hasn't been fully constructed yet.
    // serialize();  
}

then call the super class constructor from Place constructor, instead of setting the field directly:

public Place(String name) {
    super(name);
}

And finally, make your Place class to implement Serializable interface. Remember, you are serializing it's instance.

回答2:

Your Entry#main() isn't getting called because Main#main() is your main() method now. You need to add a constructor that initializes the filename in your Entry class as

   public Entry() {
        this.filename = this.name + ".ser";
    }

来源：https://stackoverflow.com/questions/18138526/nullpointerexception-when-trying-to-write-object-instance-to-objectoutputstream

标签

java

pointers

serialization

stream

nullpointerexception