ArrayList couldnt generate output

[Eric Tan]

I have some problem on the coding. I followed what you said on some post. I tried on my java page. whereby i put the codes in my page and tried it out. after i run it, it couldn't display any results from the file itself when i want to retrieve the results out.

public void RetrieveData(){
//int index =0;
db = Db4o.openFile(filename);
Query query = db.query();
query.constrain(Country.class);

ObjectSet result = query.execute();
ArrayList cntr = new ArrayList();

while( result.hasNext() ){
    Object list = result.next();
    cntr.add (list);
    System.out.println("Result = " + list );
}

//Return array;
}

this is part of the coding where the retrieve is. Maybe i am wrong in the codes that i put in. Can anyone guide me on this? Thanks

[David Nemeskey]

Hello,

Are you sure this is the right forum to ask it? First of all, this is not a general java question at all, I think it should be moved to a database-related forum. Second, it seems as if you are answering to someone; wasn't your post intended as a reply, and not a new topic?

Anyway, now for the question:
It seems to me that you accidentally swapped the name of your variables. Also, you print all countries (called "list") one-by-one, instead of the whole list.

So, didn't you want to write

public void RetrieveData() {
    db = Db4o.openFile(filename);
    Query query = db.query();
    query.constrain(Country.class);
    
    ObjectSet result = query.execute();
    List list = new ArrayList();
    
    while (result.hasNext()) {
Object cntr = result.next();
list.add(cntr);
    }
    
    System.out.println("Result = " + list );
}

The code above should output all Countries in the following way:
[<country1>, <country2>, .... <countryn>]

However, even with the original code, it should have written something, provided there are countries in the file!

So, I suggest to check if the result set is empty, like this:

public void RetrieveData() {
    db = Db4o.openFile(filename);
    Query query = db.query();
    query.constrain(Country.class);
    
    ObjectSet result = query.execute();
    
    if (result.hasNext()) {
        List list = new ArrayList();
        
        while (result.hasNext()) {
            list.add(result.next());
        }
        
        System.out.println("Result = " + list);
    } else {
        System.out.println("Result set is empty.");
    }
}

This will print "Result set is empty", if there are no countries in the db file.
[ November 17, 2006: Message edited by: David Nemeskey ]

[Eric Tan]

Thanks alot for yourr help. Sorry about the posting on the wrong forum. Because i thought it was a java problem with arraylist.
Thanks again, really appreciate it

[Eric Tan]

Thanks alot. I paste the codes and it works... Really appreciate it alot.
And sorry about the wrong posting

[David Nemeskey]

You are welcome.

However, I just had a look at the forums, and do not really know where you could have posted it... Seems like all database forums are relational database-related (O-R mapping, JDBC). So maybe we need an object DB, or at least a general persistence forum.

I have asked it in the JavaRanch forum, well, we'll see.

[Eric Tan]

Yeah. can open one too. since the world now is emphasizing a lot on object database so i think it's time to have a forum about it to recommend people the advantage of it