尝试将对象实例写入ObjectOutputStream时发生NullPointerException

Question

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.

Answer 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.

Answer 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";
    }