Java: Writing/Reading a Map from disk

Yes, your objects will need to implement Serializable in order to be serialized by the default Java mechanism. HashMap and String already implement this interface and thus can be serialized successfully.

Take a look at Sun's own Serialization tutorial - it's quite short and I think should cover everything you need for your simple case. (You should just be able to serialise the Map object to the stream, and then read it back in on subsequent runs).

If you do run into problems, try serializing a simple HashMap<String, String> with some dummy values. If this succeeds, you'll know that the problem lies (somehow) with your own class' serializability; alternatively, if this doesn't work you can focus on the basic structure before throwing your own class into the mix.

Post back if you have any more specific problems that you can't figure out on your own.

If you aren't concerned about Object particularly , you just need key value pair of String,String then I would suggest you to go for java.util.Properties. otherwise here you go

        Map map = new HashMap();
        map.put("1",new Integer(1));
        map.put("2",new Integer(2));
        map.put("3",new Integer(3));
        FileOutputStream fos = new FileOutputStream("map.ser");
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        oos.writeObject(map);
        oos.close();

        FileInputStream fis = new FileInputStream("map.ser");
        ObjectInputStream ois = new ObjectInputStream(fis);
        Map anotherMap = (Map) ois.readObject();
        ois.close();

        System.out.println(anotherMap);

Map m = new HashMap();
// let's use untyped and autoboxing just for example
m.put("One",1);
m.put("Two",2);

ObjectOutputStream oos = new ObjectOutputStream(
        new FileOutputStream("foo.ser")
);
oos.writeObject(m);
oos.flush();
oos.close();