Java: Writing/Reading a Map from disk

Question

I have a data structure that I would like to be able to write to a file before closing the program, and then read from the file to re-populate the structure the next time the application starts.

My structure is HashMap<String, Object>. The Object is pretty simple; For member variables it has a String, and two small native arrays of type Boolean. This is a real simple application, and I wouldn't expect more than 10-15 <key,value> pairs at one time.

I have been experimenting (unsuccessfully) with Object input/output streams. Do I need to make the Object class Serializable?

Can you give me any suggestions on the best way to do this? I just need a push in the right direction. Thanks!

EDIT: Well I feel dumb still, I was writing from one map and reading into another map, and then comparing them to check my results. Apparently I was comparing them wrong. Sigh.

Solution 1

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);

Solution 2

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.

Solution 3

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();