The Java driver provides a DBObject interface to save custom objects to the database.
For example, suppose one had a class called Tweet that they wanted to save:
public class Tweet implements DBObject {
/* ... */
}
Then you can say:
Tweet myTweet = new Tweet();
myTweet.put("user", userId);
myTweet.put("message", msg);
myTweet.put("date", new Date());
collection.insert(myTweet);
When a document is retrieved from the database, it is automatically converted to a DBObject. To convert it to an instance of your class, use DBCollection.setObjectClass():
collection.setObjectClass(Tweet.class);
Tweet myTweet = (Tweet)collection.findOne();
If for some reason you wanted to change the message you can simply take that tweet and save it back after updating the field.
Tweet myTweet = (Tweet)collection.findOne();
myTweet.put("message", newMsg);
collection.save(myTweet);