为什么我在Google App Engine上使用JDO在我的查询中收到了强制转换错误？

Question

根据查询和索引文档，您可以有效地进行查询（据我所知）：

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();
}

但是，我在results = (ArrayList<App>) q.execute();上得到以下错误results = (ArrayList<App>) q.execute(); 线：

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)

有任何想法吗？

Answer 1

您可以将execute结果转换为List<whatever> ，但不是特定于ArrayList<whatever> ，这是您的代码尝试执行的操作！ 为什么不直接使用List ，你被允许抽象投来，而不是ArrayList ，你发现你不能转换为具体落实？