Object Serialisation-Collection of common programming errors

I had been trying to do an object serialiation,deserialisation program.When i am giving the File Name directly in the FileInputStream ,my program runs quite finely and i was able to deserialise the object successsfully.But when i tried to use FileDialog,the serialisation program runs finely,but the deserialisation cannot be done. I am getting the following error:

java.io.FileNotFoundException: nullnull (The system cannot find the file specified)
at java.io.FileInputStream.open(Native Method)
    at java.io.FileInputStream.(Unknown Source)
    at java.io.FileInputStream.(Unknown Source)
    at package1.Deserialisation.main(Deserialisation.java:22)
Exception in thread "main" java.lang.NullPointerException
    at package1.Deserialisation.main(Deserialisation.java:43) .

Can any one help me to resolve the same ande also explain in breif the reason for it so that mistakes are not repeated..

Following is my code :

       package package1;
        public class Employee implements java.io.Serializable
        {
    private static final long serialVersionUID = 2L;

    String name;
    String address;
    int age;
    //int SSN;
    transient int SSN;    
    public void checkDetails()
    {
        System.out.println("The check details for the employee" + name + "are :");
    }

}

         Serialisation:
         package package1;
        import java.awt.FileDialog;
        import java.awt.Frame;
        import java.io.FileOutputStream;
        import java.io.IOException;
        import java.io.ObjectOutputStream;

        public class Serialization 
         {
     public static void main(String[] args)
     {
          Employee emp =new Employee();
          emp.name="vidhya";
          emp.address="XYZ";
          emp.age=26;
          emp.SSN=123456;
          try 
          {   
            FileDialog fd =new FileDialog(new Frame(),"Save As...",FileDialog.SAVE);

            fd.setVisible(true);
            String filepath=new String (fd.getDirectory()+fd.getFile());
            FileOutputStream fileout =new FileOutputStream(filepath);
    ObjectOutputStream objout=new ObjectOutputStream(fileout);
    objout.writeObject(emp);
    fileout.close();
    objout.close();
    } 
          catch (IOException e2) {
        e2.printStackTrace();
        }
            }
       }


    Deserialisation :
         package package1;
        import java.awt.FileDialog;
        import java.awt.Frame;
        import java.io.FileInputStream;
        import java.io.IOException;
       import java.io.ObjectInputStream;
       public class Deserialisation 
       {
        static Employee emp=null;
    public static void main(String[] args) 
    {
      try {  
        FileDialog fd1 =new FileDialog(new Frame(),"Open...",FileDialog.LOAD);
        String filepath = new String(fd1.getDirectory() + fd1.getFile());
        FileInputStream filein =new FileInputStream(filepath);
        ObjectInputStream objin =new ObjectInputStream(filein);
        emp=(Employee) objin.readObject();
        objin.close();
        filein.close();
        } 

        catch (IOException e) 
        {
        e.printStackTrace();
        } 
        catch (ClassNotFoundException e1)
        {
            System.out.println("Employee class not found");
            e1.printStackTrace();
            return;       
        }

        System.out.println("Employee Name : " + emp.name);
        System.out.println("Address :" + emp.address);
        System.out.println("Age : " + emp.age);
        System.out.println("SSN :" + emp.SSN);
    }

}

1. String filepath = new String(fd1.getDirectory() + fd1.getFile());
   

   This is creating a path of “nullnull” meaning both these values are null. Try using fd1.show(); before the line above.

   Edit: As pointed out by Viydha show() is deprecated and setVisible() should be used.

2. The error is in your FileDialog. The line :

   String filepath = new String (fd.getDirectory()+fd.getFile());
   

   creates a path of nullnull. You should do a fd.show(); before this line, or, better, use a JFileChooser