NullPointerException when trying to write object instance to ObjectOutputStream

NullPointerException when trying to write object instance to
ObjectOutputStream

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.