清除 JavaFX 组合框时出现空指针异常 [重复]答案

【问题标题】：Null pointer exception when clearing a JavaFX combobox [duplicate]清除 JavaFX 组合框时出现空指针异常 [重复]
【发布时间】：2018-01-31 01:46:47
【问题描述】：

我有一个组合框对象，定义如下：

ObservableList<String> workflowsComboList = FXCollections.observableArrayList();
ComboBox<String> comboBoxWorkflows = new ComboBox<String>(workflowsComboList);

workflowsComboList 列表根据用户的输入填充了某些选项。然后，当用户输入发生变化时，我需要重置列表并向组合框添加新选项。为此，我运行：

workflowsComboList.clear();

此时，如果组合框中没有选择任何值，.clear() 可以正常工作。但是，如果选择了一个值，则会引发空指针异常。

我认为错误是因为我正在删除组合框中的选定元素而引起的，所以我试图在清除列表之前取消选择选定的项目，但我尝试过的方法似乎都不起作用，包括：

comboBoxWorkflows.setValue(null);
comboBoxWorkflows.getSelectionModel().clearSelection();
comboBoxWorkflows.valueProperty().set(null);

我是否误以为这是异常的原因？或者，如果我是对的，那么在组合框中取消选择元素的正确方法是什么？

编辑：

当我调用 .clear() 时似乎发生了异常。如果我这样做：

System.out.println("List contents: " + list.toString());
try{
    System.out.println(“About to clear”);
    workflowsComboList.clear();
    System.out.println("List successfully cleared.")
catch (Exception e){
    System.out.println("Exception: " + e);
}

我明白了：

List contents: [item1, item2, item3]
About to clear.
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException...
...
List successfully cleared.

这是完整的堆栈跟踪：

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
    at javafx_uitabpane.Javafx_uitabpane$11.handle(Javafx_uitabpane.java:738)
    at javafx_uitabpane.Javafx_uitabpane$11.handle(Javafx_uitabpane.java:1)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Node.fireEvent(Node.java:8413)
    at com.sun.javafx.scene.control.skin.ComboBoxListViewSkin.handleControlPropertyChanged(ComboBoxListViewSkin.java:179)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase.lambda$registerChangeListener$61(BehaviorSkinBase.java:197)
    at com.sun.javafx.scene.control.MultiplePropertyChangeListenerHandler$1.changed(MultiplePropertyChangeListenerHandler.java:55)
    at javafx.beans.value.WeakChangeListener.changed(WeakChangeListener.java:89)
    at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:361)
    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:81)
    at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(ObjectPropertyBase.java:105)
    at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
    at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:146)
    at javafx.scene.control.ComboBoxBase.setValue(ComboBoxBase.java:150)
    at javafx.scene.control.ComboBox.updateValue(ComboBox.java:463)
    at javafx.scene.control.ComboBox.access$200(ComboBox.java:192)
    at javafx.scene.control.ComboBox$3.changed(ComboBox.java:446)
    at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:361)
    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:81)
    at javafx.beans.property.ReadOnlyObjectPropertyBase.fireValueChangedEvent(ReadOnlyObjectPropertyBase.java:74)
    at javafx.beans.property.ReadOnlyObjectWrapper.fireValueChangedEvent(ReadOnlyObjectWrapper.java:102)
    at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
    at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:146)
    at javafx.scene.control.SelectionModel.setSelectedItem(SelectionModel.java:102)
    at javafx.scene.control.ComboBox$ComboBoxSelectionModel.lambda$new$154(ComboBox.java:494)
    at com.sun.javafx.binding.ExpressionHelper$SingleInvalidation.fireValueChangedEvent(ExpressionHelper.java:137)
    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:81)
    at javafx.beans.property.ReadOnlyIntegerPropertyBase.fireValueChangedEvent(ReadOnlyIntegerPropertyBase.java:72)
    at javafx.beans.property.ReadOnlyIntegerWrapper.fireValueChangedEvent(ReadOnlyIntegerWrapper.java:102)
    at javafx.beans.property.IntegerPropertyBase.markInvalid(IntegerPropertyBase.java:113)
    at javafx.beans.property.IntegerPropertyBase.set(IntegerPropertyBase.java:147)
    at javafx.scene.control.SelectionModel.setSelectedIndex(SelectionModel.java:68)
    at javafx.scene.control.ComboBox$ComboBoxSelectionModel$2.onChanged(ComboBox.java:524)
    at javafx.collections.WeakListChangeListener.onChanged(WeakListChangeListener.java:88)
    at com.sun.javafx.collections.ListListenerHelper$Generic.fireValueChangedEvent(ListListenerHelper.java:329)
    at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:73)
    at javafx.collections.ObservableListBase.fireChange(ObservableListBase.java:233)
    at javafx.collections.ListChangeBuilder.commit(ListChangeBuilder.java:482)
    at javafx.collections.ListChangeBuilder.endChange(ListChangeBuilder.java:541)
    at javafx.collections.ObservableListBase.endChange(ObservableListBase.java:205)
    at com.sun.javafx.collections.ObservableListWrapper.clear(ObservableListWrapper.java:157)
    at javafx_uitabpane.Javafx_uitabpane$10.handle(Javafx_uitabpane.java:706)
    at javafx_uitabpane.Javafx_uitabpane$10.handle(Javafx_uitabpane.java:1)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Node.fireEvent(Node.java:8413)
    at com.sun.javafx.scene.control.skin.ComboBoxListViewSkin.handleControlPropertyChanged(ComboBoxListViewSkin.java:179)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase.lambda$registerChangeListener$61(BehaviorSkinBase.java:197)
    at com.sun.javafx.scene.control.MultiplePropertyChangeListenerHandler$1.changed(MultiplePropertyChangeListenerHandler.java:55)
    at javafx.beans.value.WeakChangeListener.changed(WeakChangeListener.java:89)
    at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:361)
    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:81)
    at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(ObjectPropertyBase.java:105)
    at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
    at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:146)
    at javafx.scene.control.ComboBoxBase.setValue(ComboBoxBase.java:150)
    at javafx.scene.control.ComboBox.updateValue(ComboBox.java:463)
    at javafx.scene.control.ComboBox.access$200(ComboBox.java:192)
    at javafx.scene.control.ComboBox$3.changed(ComboBox.java:446)
    at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:361)
    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:81)
    at javafx.beans.property.ReadOnlyObjectPropertyBase.fireValueChangedEvent(ReadOnlyObjectPropertyBase.java:74)
    at javafx.beans.property.ReadOnlyObjectWrapper.fireValueChangedEvent(ReadOnlyObjectWrapper.java:102)
    at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
    at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:146)
    at javafx.scene.control.SelectionModel.setSelectedItem(SelectionModel.java:102)
    at javafx.scene.control.ComboBox$ComboBoxSelectionModel.lambda$new$154(ComboBox.java:494)
    at com.sun.javafx.binding.ExpressionHelper$SingleInvalidation.fireValueChangedEvent(ExpressionHelper.java:137)
    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:81)
    at javafx.beans.property.ReadOnlyIntegerPropertyBase.fireValueChangedEvent(ReadOnlyIntegerPropertyBase.java:72)
    at javafx.beans.property.ReadOnlyIntegerWrapper.fireValueChangedEvent(ReadOnlyIntegerWrapper.java:102)
    at javafx.beans.property.IntegerPropertyBase.markInvalid(IntegerPropertyBase.java:113)
    at javafx.beans.property.IntegerPropertyBase.set(IntegerPropertyBase.java:147)
    at javafx.scene.control.SelectionModel.setSelectedIndex(SelectionModel.java:68)
    at javafx.scene.control.SingleSelectionModel.updateSelectedIndex(SingleSelectionModel.java:215)
    at javafx.scene.control.SingleSelectionModel.select(SingleSelectionModel.java:149)
    at com.sun.javafx.scene.control.skin.ComboBoxListViewSkin.lambda$createListView$323(ComboBoxListViewSkin.java:484)
    at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:349)
    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:81)
    at javafx.beans.property.ReadOnlyIntegerPropertyBase.fireValueChangedEvent(ReadOnlyIntegerPropertyBase.java:72)
    at javafx.beans.property.ReadOnlyIntegerWrapper.fireValueChangedEvent(ReadOnlyIntegerWrapper.java:102)
    at javafx.beans.property.IntegerPropertyBase.markInvalid(IntegerPropertyBase.java:113)
    at javafx.beans.property.IntegerPropertyBase.set(IntegerPropertyBase.java:147)
    at javafx.scene.control.SelectionModel.setSelectedIndex(SelectionModel.java:68)
    at javafx.scene.control.MultipleSelectionModelBase.select(MultipleSelectionModelBase.java:404)
    at javafx.scene.control.MultipleSelectionModelBase.clearAndSelect(MultipleSelectionModelBase.java:356)
    at javafx.scene.control.ListView$ListViewBitSetSelectionModel.clearAndSelect(ListView.java:1403)
    at com.sun.javafx.scene.control.behavior.CellBehaviorBase.simpleSelect(CellBehaviorBase.java:256)
    at com.sun.javafx.scene.control.behavior.CellBehaviorBase.doSelect(CellBehaviorBase.java:220)
    at com.sun.javafx.scene.control.behavior.CellBehaviorBase.mousePressed(CellBehaviorBase.java:150)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:95)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
    at com.sun.javafx.event.CompositeEventHandler$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$MouseHandler.process(Scene.java:3757)
    at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
    at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:381)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$354(GlassViewEventHandler.java:417)
    at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:416)
    at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
    at com.sun.glass.ui.View.notifyMouse(View.java:937)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
    at java.lang.Thread.run(Unknown Source)

【问题讨论】：

在问题中发布完整的stack trace，并确定代码中抛出它的行。
请edit 在您的问题中包含产生堆栈跟踪的minimal reproducible example。
你意识到除非你真的告诉我们，否则你是唯一知道哪一行是第 738 行的人，对吧？
workflowsComboList.clear() 可以是引发异常的实际行的唯一方法是如果workflowsComboList 为空。真的不清楚为什么你不会明确告诉我们哪一行是引发异常的行。如果你不这样做，除了猜测之外，没有人可以做任何事情。
愚弄这个。它吸引了疯狂的（在某些情况下是非常不正确的）猜测作为“答案”，这有可能在未来误导其他用户。尽管被特别要求提供基本信息，但 OP 似乎奇怪地不愿提供基本信息。问题中没有足够的信息，因为它可以改进一个答案，证明关于出了什么问题的假设是不正确的。

标签： javafx combobox nullpointerexception

【解决方案1】：

如果您将 ComboBox Selection 绑定到另一个变量，请确保您处理的是 Null 值。

【讨论】：

【解决方案2】：

我认为您可能需要从组合框中删除可观察列表

   comboBoxWorkflows .getItems().removeAll( workflowsComboList);

【讨论】：

这相当于workflowsComboList.removeAll(workflowsComboList);。充其量这相当于 OP 已经发布的代码 - 最坏的情况是它会抛出一个 ConcurrentModificationException。
实际上我之前尝试过，正如 James_D 所说，我遇到了同样的异常。

【解决方案3】：

我认为错误是因为我删除了ComboBox 中的选定元素。

您可能需要重新检查这个假设。从这个example 开始，我在ObservableList 模型中添加了以下按钮clear 和restore。由于没有发生错误，无论选择如何，您都需要在其他地方查找异常原因。

Button clear = new Button("Clear");
clear.setOnAction((action) -> {
    options.clear();
});
Button restore = new Button("Restore");
restore.setOnAction((action) -> {
    options.clear();
    options.addAll(Venue.values());
    comboBox.getSelectionModel().selectLast();
});
dialogPane.setContent(new VBox(8, textField, datePicker, comboBox, clear, restore));

【讨论】：

非常感谢您的洞察力。奇怪的是，当我调用 .clear() 时，异常似乎确实发生了。我已经用更多信息和完整的堆栈跟踪编辑了主要帖子。