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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!

2 Answers2

3

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

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Peter Lawrey
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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
}