Question

I am trying to query the data store and cast it to a user defined object type. But, i am getting an class cast error. Please look into the code

   import com.gwt.samples.shared.List;

   public ArrayList<String> viewLists(String user_id) {
    PersistenceManager pm = PMF.get().getPersistenceManager();
    ArrayList<String> res;
    String[] ret;
            Query q=pm.newQuery(List.class);
            q.setFilter("owner_id == useridparam");
            q.declareParameters("String useridparam");

            try
            {
               res=(ArrayList<List>)q.execute(user_id); //error occurs here i guess
            }
            finally
            {
               q.closeAll();
            }
            return res;
         }

The problem occurs because query.execute returns an Object and it cannot be casted to the type ArrayList . But, i am following this example from here

Please help

Answer 1

May I suggest:

1.Delete the line

import com.gwt.samples.shared.List;

2.Instead of

Query q=pm.newQuery(List.class);

and

res=(ArrayList<List>)q.execute(user_id); //error occurs here i guess

use

Query q=pm.newQuery(com.gwt.samples.shared.List.class);

and

res=(ArrayList<java.util.List>)q.execute(user_id);

When you say “error occurs here i guess”, I presume that you can find out definitely by examining the relevant exception stack trace or log entry.

Also, I have just seen that your data member res is of data type ArrayList<String>, whereas your query will return (incorporating my code changes above) java.util.List<com.gwt.samples.shared.List>.

You will need to change the data type of either your query or res to get things to work.

I have a standard method for querying. The query return is stored in a java.util.List. I then do

java.util.List liResult = [My query];
ArrayList alResult = new ArrayList(liResult);

Ouside of my method, I cast my ArrayList to ArrayList<[Query data type]>.

Any help?