lambda表达式中的空指针异常

我有一个JavaFx应用程序，其中有一个名为roomList的ListView。我使用以下代码行检查是否从ListView中选择了某些内容：

    roomList.getSelectionModel().selectedItemProperty().addListener((observable, oldValue, newValue)   -> showRoomElements(newValue));

如果选择了某个项目，那么根据选择的项目，我会更改某些标签上的一些文本。那很好。 当我使用其他菜单更改roomList中的项目时，问题出现了。它仍然可以更改标签，但我有空指针异常

    Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at homeautomation.view.HomePageController.showRoomElements(HomePageController.java:207)
at homeautomation.view.HomePageController.lambda$setHomeAutomation$1(HomePageController.java:59)
at homeautomation.view.HomePageController$$Lambda$276/1011775954.changed(Unknown Source)
at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:182)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:81)
at javafx.beans.property.ReadOnlyObjectWrapper$ReadOnlyPropertyImpl.fireValueChangedEvent(ReadOnlyObjectWrapper.java:176)
at javafx.beans.property.ReadOnlyObjectWrapper.fireValueChangedEvent(ReadOnlyObjectWrapper.java:142)
at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:145)
at javafx.scene.control.SelectionModel.setSelectedItem(SelectionModel.java:102)
at javafx.scene.control.MultipleSelectionModelBase.lambda$new$34(MultipleSelectionModelBase.java:67)
at javafx.scene.control.MultipleSelectionModelBase$$Lambda$192/1369453379.invalidated(Unknown Source)
at com.sun.javafx.binding.ExpressionHelper$SingleInvalidation.fireValueChangedEvent(ExpressionHelper.java:137)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:81)
at javafx.beans.property.ReadOnlyIntegerWrapper$ReadOnlyPropertyImpl.fireValueChangedEvent(ReadOnlyIntegerWrapper.java:176)
at javafx.beans.property.ReadOnlyIntegerWrapper.fireValueChangedEvent(ReadOnlyIntegerWrapper.java:142)
at javafx.beans.property.IntegerPropertyBase.markInvalid(IntegerPropertyBase.java:113)
at javafx.beans.property.IntegerPropertyBase.set(IntegerPropertyBase.java:146)
at javafx.scene.control.SelectionModel.setSelectedIndex(SelectionModel.java:68)
at javafx.scene.control.ListView$ListViewBitSetSelectionModel.updateItemsObserver(ListView.java:1287)
at javafx.scene.control.ListView$ListViewBitSetSelectionModel.access$000(ListView.java:1186)
at javafx.scene.control.ListView$1.invalidated(ListView.java:426)
at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:111)
at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:145)
at javafx.scene.control.ListView.setItems(ListView.java:394)
at homeautomation.view.HomePageController.handleSelectFirstFloor(HomePageController.java:141)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1759)
at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1645)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.doSelect(ContextMenuContent.java:1364)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.lambda$createChildren$324(ContextMenuContent.java:1317)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer$$Lambda$295/2103054710.handle(Unknown Source)
at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3724)
at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3452)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1728)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2461)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:348)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:273)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:382)
at com.sun.glass.ui.View.handleMouseEvent(View.java:553)
at com.sun.glass.ui.View.notifyMouse(View.java:925)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$141(WinApplication.java:102)
at com.sun.glass.ui.win.WinApplication$$Lambda$37/728890494.run(Unknown Source)
at java.lang.Thread.run(Thread.java:745)

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正如我在评论中所建议的，我建议重构代码，以便尽可能使用绑定而不是侦听器

假设你有一间教室，像这样：

class Room {
    String getFoo();
    StringProperty barProperty();
}

并且您希望分别在

doubabel

和

barLabel

中显示所选房间的“foo”和“bar”属性。我将使用创建从“selected room”属性派生的绑定

// wrap the selectedItemProperty to get the map and flatMap methods used below
MonadicObservableValue<Room> selectedRoom = EasyBind.monadic(
        roomList.getSelectionModel().selectedItemProperty());

fooLabel.textProperty().bind(selectedRoom.map(Room::getFoo));
barLabel.textProperty().bind(selectedRoom.flatMap(Room::barProperty));

绑定是更好的样式 与监听器相比，更喜欢绑定是一种更好的风格，因为绑定更具声明性地说明了

傻瓜标签的文本是什么。这里写着

傻瓜床
显示所选房间的“foo”属性，或文本“未选择房间”

将此与侦听器方法进行比较，其中

swookabel
从代码的任何部分显示它被告知要显示的内容

换句话说，每个侦听器都是一个副作用，副作用会导致无法读取的意大利面代码。
当您取消选择ListView中的所有项目时（例如，完全更改项目），则侦听器中的
newValue
为
null
，导致NullPointerException的原因。是否有办法解决此问题？只需在
showRoomElements
方法中检查null。如果可能，我还建议使用绑定而不是侦听器。为此，我必须知道showRoomElements

正在做什么。showRoomElements只是在更改标签上的文本谢谢，我将使用这个。

fooLabel.textProperty().bind(
        selectedRoom.map(Room::getFoo).orElse("No room selected"));