Why am I getting a cast error in my Query using JDO on Google App Engine?

Question

According to the Queries and Indexes doc you can go a query effectively identically (so far as I can tell) to this: ``` PersistenceManager pm = PMF.get().getPersistenceManager(); try { Query q = pm.newQuery(App.class); q.setOrdering("name desc"); try { results = (ArrayList) q.execute(); } finally { q.closeAll(); } } finally { pm.close(); } ``` However, I'm getting the following error on the `results = (ArrayList) q.execute(); ` line: ``` java.lang.ClassCastException: org.datanucleus.store.appengine.query.StreamingQueryResult cannot be cast to java.util.ArrayList at com.killerrobots.AppSite.MainServlet.getApplications(MainServlet.java:66) at com.killerrobots.AppSite.MainServlet.doGet(MainServlet.java:26) at javax.servlet.http.HttpServlet.service(HttpServlet.java:693) at javax.servlet.http.HttpServlet.service(HttpServlet.java:806) at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHa…

Accepted Answer

You can cast the result of `execute` to a `List`, but not specifically to an `ArrayList` which is what your code is trying to do! Why not just use `List`, the abstraction you're allowed to cast to, rather than `ArrayList`, the concrete implementation that you're not allowed to cast to?

Why am I getting a cast error in my Query using JDO on Google App Engine?

Solution

Problem