ClassCastException: cannot assign instance of

Question

I write Graph object into file. Graph class is as...

import java.io.Serializable;
import java.util.ArrayList;

public class Graph implements Serializable{

    public String gid = "";
    public ArrayList<Vertex> vertices = new ArrayList<Vertex>();
    public ArrayList<Edge> edges = new ArrayList<Edge>();
    public Graph()
    {

    }
    public Graph(String id, ArrayList<Vertex> ver, ArrayList<Edge> ed)
    {
        gid = id;
        vertices = ver;
        edges = ed;
    }
    public String getId()
    {
        return gid;
    }
    public void Clear()
    {
        vertices.clear();
        edges.clear();
    }
}

The Vertex and Edge class are as follow...

import java.io.Serializable;

public class Vertex implements Serializable{

    public int vertexId;
    public String vertexLabel;
    public Vertex()
    {
        vertexId = 0;
        vertexLabel = "";
    }
    public Vertex(int id, String label)
    {
        vertexId = id;
        vertexLabel = label;
    }
    public String getVertexLabel()
    {
        return vertexLabel;
    }
    public String toString()
    {
        return vertexId + "" + vertexLabel;
    }
}


import java.io.Serializable;
public class Edge implements Serializable{
    public int sid;
    public int tid;
    public Vertex src;
    public Vertex tar;
    public String bondtype;
    int bt;
    public Edge()
    {
        sid = 0;
        tid = 0;
        src = new Vertex();
        tar = new Vertex();
        bondtype = "";
        bt = 0;
    }
    public Edge(int sourceId,int targetId,Vertex source,Vertex target,int bt)
    {
        sid = sourceId;
        tid = targetId;
        src = source;
        tar = target;
        this.bt = bt;
        if(bt == 1)
            bondtype = "s";
        else if(bt == 2)
            bondtype = "d";
        else
            bondtype = "t";
    }
    public String toString()
    {
        return src.vertexLabel + "," + bondtype + "," + tar.vertexLabel;
    }
    public String reverseString()
    {
        return tar.vertexLabel + "," + bondtype + "," + src.vertexLabel;
    }
    public String toNumString()
    {
        return src.vertexId + "," + bondtype + "," + tar.vertexId;
    }
    public int getSourceId(Edge edge)
    {
        int source = 0;
        source = edge.sid;
        return source;
    }
    public int getTargetId(Edge edge)
    {
        int target = 0;
        target = edge.tid;
        return target;
    }
}

I write and read Graph object to/from file as

public static ArrayList<Graph> ReadGraphs(String filename)
    {
        ArrayList<Graph> graphs = new ArrayList<Graph>();
        ObjectInputStream ois = null;
        Graph obj = new Graph();
        try{
             ois = new ObjectInputStream(new BufferedInputStream(new FileInputStream(filename), 16 * 1024));
             while( (obj = (Graph) ois.readObject()) != null) 
            {   
                if (obj instanceof Graph) 
                {
                    graphs.add(obj);
                }
            }
         }catch (EOFException ex) {  //This exception will be caught when EOF is reached
             //System.out.println("End of file reached.");
         } catch (ClassNotFoundException ex) {
             ex.printStackTrace();
         } catch (FileNotFoundException ex) {
             ex.printStackTrace();
         } catch (IOException ex) {
             ex.printStackTrace();
         } finally {
             try {
                 if (ois != null) {
                     ois.close();
                 }
             } catch (IOException ex) {
                 ex.printStackTrace();
             }
         }
        return graphs;
    }


public static void writeG(Graph g) throws IOException
    {
        File myFile = new File("./Graphs.txt");
        int bufferSize = 16 * 1024;
        if(!myFile.exists())
        {
            myFile.createNewFile();
            FileOutputStream fos = new FileOutputStream(myFile);
            ObjectOutputStream os = new ObjectOutputStream(new BufferedOutputStream(fos,bufferSize));
            try
             {
                os.writeObject(g);
             }
             catch (IOException e)
             {
                e.printStackTrace();
             }
            os.flush();
            os.close();
          }
        else
        {
            FileOutputStream fos =  new FileOutputStream(myFile,true);
            NoHeaderObjectOutputStream oos = new NoHeaderObjectOutputStream(new BufferedOutputStream(fos, bufferSize));
            try
             {
                oos.writeObject(g);
             }
             catch (IOException e)
             {
                e.printStackTrace();
             }
          oos.flush();
          oos.close();
        }
    }

When i run it I get following class cast exception. Here is the stack trace of the problem I'm encountering:

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: cannot assign instance of java.lang.String to field InputGraphPhase.Edge.src of type InputGraphPhase.Vertex in instance of InputGraphPhase.Edge
    at java.io.ObjectStreamClass$FieldReflector.setObjFieldValues(ObjectStreamClass.java:2083)
    at java.io.ObjectStreamClass.setObjFieldValues(ObjectStreamClass.java:1261)
    at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1996)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1915)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1798)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1350)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:370)
    at java.util.ArrayList.readObject(ArrayList.java:771)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:1017)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1893)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1798)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1350)
    at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1990)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1915)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1798)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1350)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:370)
    at InputGraphPhase.GraphParser.ReadGraphs(GraphParser.java:501)
    at GUI.MyFrame.check(MyFrame.java:1135)
    at GUI.MyFrame.showResult(MyFrame.java:1328)
    at GUI.MyFrame$6.actionPerformed(MyFrame.java:671)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6505)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
    at java.awt.Component.processEvent(Component.java:6270)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4861)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2719)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:694)
    at java.awt.EventQueue$3.run(EventQueue.java:692)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:708)
    at java.awt.EventQueue$4.run(EventQueue.java:706)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Java serialization does not produce "text" files - and be careful to not change classes or you won't be able to read back data. — Has QUIT--Anony-Mousse
– Has QUIT--Anony-Mousse, Commented Nov 12, 2015 at 6:46

user207421 · Accepted Answer · 2015-11-12 06:33:34Z

2

cannot assign instance of java.lang.String to field InputGraphPhase.Edge.src of type InputGraphPhase.Vertex in instance of InputGraphPhase.Edge

You've change the Edge class since you serialized it. When you serialized it, src was a String; when you deserialized it it was declared as a Vertex.
This loop isn't valid:
```
while( (obj = (Graph) ois.readObject()) != null)
```
readObject() doesn't return null at end of stream. It throws EOFException. A null can occur anywhere in the stream, any time you serialize one.
A serialized file is not a text file. It is binary.

edited Nov 12, 2015 at 6:33

answered Nov 12, 2015 at 6:28

user207421

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