Write arraylist with different objects to file

Question

i'm trying to write an ArrayList, with different objects in it, to a file. This is my ArrayList:

ArrayList<Person> personList = new ArrayList<Person>();
    private Person theCaptain;


    void init(){

    //Adding persons to the list

    personList.add(new Coach("Tesan de Boer", "Vieruslaan 3b", "0689337554"));
    personList.add(new Goalkeeper("Peter Post", "Eeuwige student 66", "2222", 1));
    personList.add(new Goalkeeper("piet puk", "Weg van ongenade 88", "2222", 21));
    personList.add(new Goalkeeper("Siem van Aanhoolt", "Straatweg 45", " 0612213446", 31));
    personList.add(new Fielder("Koen Weegink", "Straatweg 45", "2222", 2));
    personList.add(new Fielder("Jan-Willem Rufus op den Haar", "Straatweg 45", " 0614226698", 3));
    personList.add(new Fielder("Tom Kraniker", "Straatweg 45", "069873663", 4));
    personList.add(new Fielder("Leon het Kanon", "Straatweg 45", "2222", 6));
    personList.add(new Fielder("Robin Hogezant", "Straatweg 45", "2222", 7));
    personList.add(new Fielder("Loesoe de Kat", "Straatweg 45", "222", 8));
    personList.add(new Fielder("Morris de Spee", "Straatweg 45", "2222", 9));
    personList.add(new Fielder("Rein Zoekers", "Straatweg 45", "2222", 10));
    personList.add(new Fielder("Darion Pok", "Straatweg 45", "2222", 11));
    personList.add(new Fielder("Achmed de Bom", "Straatweg 45", "2222", 12)); 

    }

And here i'm trying to write that ArrayList to a file called test:

public class Main {

public static void main(String[] args)throws Exception {

    Team team = new Team();
    team.init();
    ArrayList<Person> playersData = team.getPersonList();

    try {

       // create a new file with an ObjectOutputStream
       FileOutputStream out = new FileOutputStream("test.txt");
       ObjectOutputStream oout = new ObjectOutputStream(out);

       // write something in the file
       oout.writeObject(playersData);

       // close the stream
       oout.close();

       // create an ObjectInputStream for the file we created before
       ObjectInputStream ois = new ObjectInputStream(new FileInputStream("test.txt"));



    } catch (Exception ex) {
       ex.printStackTrace();
    }

}

}

When i run the program i get this error:

java.io.NotSerializableException: Coach
at java.io.ObjectOutputStream.writeObject0(Unknown Source)
at java.io.ObjectOutputStream.writeObject(Unknown Source)
at java.util.ArrayList.writeObject(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at java.io.ObjectStreamClass.invokeWriteObject(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.writeObject(Unknown Source)
at Main.main(Main.java:24)

I thought ArrayList implements Serializable, so I don't know why a NotSerializableException is thrown. I hope you guys see why this exception is thrown.

Already thanks!

Answer 1

Since your ArrayList contains Coach instances, you simply have to make the Coach class implements java.io.Serializable (and probably other class you're using like Goalkeeper or Fielder).

The Serializable Javadoc says :

When traversing a graph, an object may be encountered that does not support the Serializable interface. In this case the NotSerializableException will be thrown and will identify the class of the non-serializable object.

Still, the way Serialization works in Java is rather complex. For complete information, see http://docs.oracle.com/javase/6/docs/platform/serialization/spec/serialTOC.html