Why am I getting a cast message in my request using JDO in the Google App Engine?

Question

According to query and doc indices you can make a query effectively the same (as far as I can tell):

PersistenceManager pm = PMF.get().getPersistenceManager(); try { Query q = pm.newQuery(App.class); q.setOrdering("name desc"); try { results = (ArrayList<App>) q.execute(); } finally { q.closeAll(); } } finally { pm.close(); } 

However, I get the following error in the line results = (ArrayList<App>) q.execute(); :

 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$CachedChain.doFilter(ServletHandler.java:1093) at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084) at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:360) at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216) at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181) at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:712) at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405) at com.google.apphosting.utils.jetty.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:54) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:139) at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:306) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:139) at org.mortbay.jetty.Server.handle(Server.java:313) at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:506) at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:830) at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:514) at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211) at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:381) at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:396) at org.mortbay.thread.BoundedThreadPool$PoolThread.run(BoundedThreadPool.java:442) 

Any ideas?

Answer 1

You can pass the result of execute to List<whatever> , but not specifically for ArrayList<whatever> , which is what your code does! Why not just use a List , an abstraction that you can overlay, rather than an ArrayList , a concrete implementation that you cannot use?