NotSerializable error while trying to write arraylist

Question

I am getting a NotSerializable error while trying to write an arraylist to a file.
Here's the part of the code causing problem
File temp = fileOpenerAndSaver.getSelectedFile(); // fileOpenerAndSaver is a JFileChooser object which has been created and initialized and was already used to select a file with
currentWorkingFile = new File (temp.getAbsolutePath() + ".gd");      // currentWorking file is a file object
try
     ObjectOutput output = new ObjectOutputStream (new BufferedOutputStream(new FileOutputStream(currentWorkingFile)));
     output.writeObject(nodeList); // <-- This is the line causing problem (it is line 1475 on which exception is thrown)
     output.writeObject(edgeList);
     output.writeObject(nodesWithSelfLoop);
     output.writeObject(nextId);
     output.writeUTF(currentMessage);
     output.close();
catch (Exception e2)
     JOptionPane.showMessageDialog (graphDrawer, "Unknown error writing.", "Error", JOptionPane.ERROR_MESSAGE);
     e2.printStackTrace();
} As far as what nodeList is -- it's an arraylist of my own class Node which has been serialized and any object used inside the class has been serialized as well.
Here is the declaration of nodeList:
     private ArrayList <Node> nodeList; // ArrayList to hold a list all the nodesLet me show you whats inside the node class:
private class Node implements Serializable
     private static final long serialVersionUID = -4625153386839971250L;
     /*  edgeForThisNodeList holds all the edges that are between this node and another node
      *  nodesConnected holds all the nodes that are connected (adjacent) to this node
      *  p holds the top left corner coordinate of this node.  The centre. of this node is at (p.x + 5, p.y + 5)
      *  hasSelfLoop holds whether this node has a self loop.
      *  **NOTE**: ONLY ONE SELF LOOP IS ALLOWED FOR A NODE.
     ArrayList <Edge> edgeForThisNodeList = new ArrayList <Edge> ();
     ArrayList <Node> nodesConnected = new ArrayList <Node> ();
     Point p;
     boolean hasSelfLoop = false;
     int index = -1;
     BigInteger id;
                 ... some methods following this....
}Here is the edge class
private class Edge implements Serializable
     private static final long serialVersionUID = -72868914829743947L;
     Node p1, p2; // The two nodes
     Line2D line;
      * Constructor:
      * Assigns nodes provided as appropriate.
      * Also calls the addEdge method on each of the two nodes, and passes
      * "this" edge and the other node to the nodes, so that
      * this edge and the other node can be added to the
      * data of the node.
     public Edge (Node p1, Node p2)
          this.p1 = p1;
          this.p2 = p2;
          line = new Line2D.Float(p1.p.x+5,p1.p.y+5,p2.p.x+5,p2.p.y+5);
          p1.addEdge(this, p2, true);
          p2.addEdge(this, p1, false);
     }Here is the error I am getting:
java.io.NotSerializableException: javax.swing.plaf.metal.MetalFileChooserUI
     at java.io.ObjectOutputStream.writeObject0(Unknown Source)
     at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
     at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
     at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
     at java.io.ObjectOutputStream.writeObject0(Unknown Source)
     at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
     at java.io.ObjectOutputStream.writeObject0(Unknown Source)
     at java.io.ObjectOutputStream.writeObject(Unknown Source)
     at MyPanel.save(MyPanel.java:1475)
     at GraphDrawer.actionPerformed(GraphDrawer.java:243)
     I inentionally did not write the whole error stack because it was way too long and I ran out of characters. But line 1475 is where I do the writeObject call. I also goet some weird not serialized exceptions before on some other classes that are not being written to the file or are not part of the object that I am writing. I serialized those classes and now it's this unknown object that's causing problems.
Edit: The problem may be due to the JFileChooser I am using to select the file as it is a non-serializable object. But I am not trying to write it to the file and in fact am just writing the arrayLists and BigInteger as objects to the file.
Edited by: Cricket_struck on Dec 21, 2009 1:25 PM
Edited by: Cricket_struck on Dec 21, 2009 1:32 PM
Edited by: Cricket_struck on Dec 21, 2009 2:16 PM

Answer 1

java.io.NotSerializableException: javax.swing.plaf.metal.MetalFileChooserUIThat's a Swing component and it is clearly related to the JFileChooser.
I also get some weird not serialized exceptions before on some other classes that are not being written to the file or are not part of the object that I am writing.That's a contradiction in terms. If you get NotSerialzableException it means the objects are being written. So they are reachable from the objects you're writing.
Edit: The problem may be due to the JFileChooser I am using to select the file as it is a non-serializable object.JFileChooser implements Serializable, but you're right, you shouldn't try to serialize it. But clearly this is the current problem. Somewhere you have a reference to it reachable via the object(s) you are serializing.
I suspect that Node and Edge inner classes and that you aren't aware that inner classes contain a hidden reference to their containing class. The solution for serializaition purposes is to make them static nested classes, or outer classes.