GWT - Warum diese Geschichte nicht funktioniert?

Question

Ich bin versucht, die Geschichte auf GWT. Ich finde diesen approch wirklich schön zu implementieren. Leider, mein Code funktioniert nicht.

Das ist mein eigentlicher Code:

public class PageLoader implements EntryPoint, ValueChangeHandler<String> {
    private FlowPanel pageloader;
    private GWTServiceAsync rpcService;

    public void onModuleLoad() {
        pageloader=new FlowPanel();
        rpcService=GWT.create(GWTService.class);
        pageloader.add(new HomepageContext(rpcService));
        RootPanel.get().add(pageloader);

        History.addValueChangeHandler(this);
        if(!History.getToken().isEmpty()){
            changePage(History.getToken());
        }
    }

    public void onValueChange(ValueChangeEvent event) {
        changePage(History.getToken());
    }

    public void changePage(String token) {
        if(History.getToken().equals("apartment")) {
            pageloader.clear();
            pageloader.add(new ApartmentContext(rpcService));
        } else {
            pageloader.clear();
            pageloader.add(new HomepageContext(rpcService));
        }
    }
}

############################################################

public class ApartmentContext extends Index {
    public ApartmentContext(GWTServiceAsync rpcService) {
        super(rpcService);
        this.getContent().add(new Apartment());
    }
}    

public class Apartment extends FlowPanel {
    // somethings
}

############################################################

public class Index extends FlowPanel {
    private Center center;
    public Index(GWTServiceAsync rpcService) {
        this.center=new Center(rpcService);
        this.add(center);
   }
   public FlowPanel getContent() {
       return center.getContent();
   }
} 

############################################################

public class Center extends FlowPanel {
    private Menu menu;
    private Content content;

    public Center(GWTServiceAsync rpcService) {
       this.content=new Content();
       this.menu=new Menu(rpcService, this);

       this.add(menu);
       this.add(content);
    }

    public Content getContent() {
        return this.content;
    }
}  

Wenn ich auf Last #apartment versuchen, erhalte ich:

01:19:45,112 [ERROR] Uncaught exception escaped
java.lang.NullPointerException: null    
at org.sinfonet.client.apartment.Apartment.<init>(Apartment.java:24)    
at org.sinfonet.client.context.ApartmentContext.<init>(ApartmentContext.java:14)    
at org.sinfonet.client.PageLoader.changePage(PageLoader.java:36)    
at org.sinfonet.client.PageLoader.onValueChange(PageLoader.java:30)     
at com.google.gwt.event.logical.shared.ValueChangeEvent.dispatch(ValueChangeEvent.java:128)     
at com.google.gwt.event.logical.shared.ValueChangeEvent.dispatch(ValueChangeEvent.java:1)   
at com.google.gwt.event.shared.HandlerManager$HandlerRegistry.fireEvent(HandlerManager.java:65)     
at com.google.gwt.event.shared.HandlerManager$HandlerRegistry.access$1(HandlerManager.java:53)  
at com.google.gwt.event.shared.HandlerManager.fireEvent(HandlerManager.java:178)    
at com.google.gwt.user.client.impl.HistoryImpl.fireEvent(HistoryImpl.java:74)   
at com.google.gwt.event.logical.shared.ValueChangeEvent.fire(ValueChangeEvent.java:43)  
at com.google.gwt.user.client.impl.HistoryImpl.fireHistoryChangedImpl(HistoryImpl.java:81)  
at com.google.gwt.user.client.impl.HistoryImpl.newItem(HistoryImpl.java:118)    
at com.google.gwt.user.client.History.newItem(History.java:173)     
at com.google.gwt.user.client.History.newItem(History.java:156)     
at com.google.gwt.user.client.ui.Hyperlink.onBrowserEvent(Hyperlink.java:163)   
at com.google.gwt.user.client.DOM.dispatchEventImpl(DOM.java:1307)  
at com.google.gwt.user.client.DOM.dispatchEvent(DOM.java:1263)  
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)  
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)    
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)    
at java.lang.reflect.Method.invoke(Method.java:597)     
at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)    
at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)   
at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:157)    
at com.google.gwt.dev.shell.BrowserChannel.reactToMessagesWhileWaitingForReturn(BrowserChannel.java:1714)   
at com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:165)    
at com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:120)    
at com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:507)  
at com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:264)    
at com.google.gwt.dev.shell.JavaScriptHost.invokeNativeObject(JavaScriptHost.java:91)   
at com.google.gwt.core.client.impl.Impl.apply(Impl.java)    
at com.google.gwt.core.client.impl.Impl.entry0(Impl.java:188)   
at sun.reflect.GeneratedMethodAccessor131.invoke(Unknown Source)    
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)    
at java.lang.reflect.Method.invoke(Method.java:597)     
at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)    
at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)   
at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:157)    
at com.google.gwt.dev.shell.BrowserChannel.reactToMessages(BrowserChannel.java:1669)    
at com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:401)   
at com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:222)     
at java.lang.Thread.run(Thread.java:619)

1 - Wenn ich gut verstehen, wenn ich History.newItem () verwenden, ich nenne das onValueChange () richtig? Warum also nicht?

2 - Die Klasse Geschichte statisch ist? Also ich muss es keine Zeit verstreichen; und ich kann es in jedem Wettbewerb meiner Bewerbung nennen?

Answer 1

auf change () -Methode, versuchen Sie die Token-Variable aus dem Argument:

public void change (String Token)
{
  if ( "Wohnung" .equals (Token)) {...}
  else {...}
}

nur knapp sein Ziel bemerkt man eine Ausnahme hatte ... wo hat diese Ausnahme geschehen? (Code line ..)