How to write non-serialized objects, such as Shapes, in a file so they can then be read later

Question

I have a program with a class similar to the basic example below:

public class Vertex extends Circle implements Serializable {
    private int vertexID;
    private final static double SIZE = 10.0;

    // Constructor
    public Vertex(int ID, double x, double y) {
        super(x, y, SIZE, Color.BLUE);
        this.vertexID = ID;
    }
}

What I'd like to be able to do is write a List<Vertex> myVertices to a file using the ObjectOutputStream , which requires that each class implements the Serializable interface. An example is shown below:

FileChooser fc = new FileChooser();
File fileChosen = fc.showOpenDialog(window);

try {               
   FileOutputStream fOutStream = new FileOutputStream(fileChosen);
   ObjectOutputStream oOutStream = new ObjectOutputStream(fOutStream);

   oOutStream.writeObject(myVertices);
   oOutStream.close();
} catch {
    // Exception handling here
}

The problem with the implementation above is that, although Vertex implements Serializable , the super class Circle and its super class Shape do not implement Serializable . The result is that the file contains the Vertex objects but all Shape details are lost and default to 0.

Is there a simple solution to this sort of problem? My only current option seems to be to create my own Shape / Circle that stores the location/display data as doubles so that it can then be Serializable . Obviously for one class this isn't too much effort, but I have a few other Shape objects I'd like to save as well. I would, I assume, then have to either construct actual Shape objects to display them.

Thanks!

Answer 1

You need to create a writeObject/readObject method which saves the additional state that you need.

Serialization - readObject writeObject overrides

Uses of readObject/writeObject in Serialization

Why are readObject and writeObject private, and why would I write transient variables explicitly?

Java Custom Serialization

Java: efficiency of writeObject vs writeExternal

Answer 2

In case somebody wants to see the actual code for a Shape , specifically Circle object, here is what I implemented thanks to the accepted answer. Also included an example of a List<Vertex> myVertices = new ArrayList() :

private void writeObject(ObjectOutputStream oos) throws IOException {
    oos.writeInt(vertexID);
    oos.writeObject(myVertices);
    oos.writeDouble(super.getCenterX());
    oos.writeDouble(super.getCenterY());
}

private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
    // Read in same order as write
    this.vertexID = ois.readInt();
    this.myVertices = (List<Vertex>) ois.readObject();
    super.setCenterX(ois.readDouble());
    super.setCenterY(ois.readDouble());

    // Default Circle properties
    super.setRadius(SIZE); // Default
    super.setFill(Color.BLUE); // Default
}