空指針異常清除一個JavaFX組合框

-2

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

的workflowsComboList列表中填充根據用戶的輸入的某些選項。然後，當用戶輸入更改時，我需要重置列表並向組合框添加新選項。爲了做到這一點我運行：

workflowsComboList.clear();

在這一點上，如果沒有在組合框中選擇的值，.clear（）工作正常。但是，如果選擇了某個值，則會引發空指針異常。

我假設錯誤是由於我刪除了組合框中的選定元素而導致的，因此我試圖在清除列表之前取消選擇所選項目，但是我所嘗試的方法似乎都不起作用，包括：

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

我誤認爲這是例外的原因嗎？或者，如果我是對的，在組合框中取消選擇元素的正確方法是什麼？

編輯：

的例外似乎發生時，我調用.clear（）。如果我做的：

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); 
}

我得到：

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

下面是完整的堆棧跟蹤：

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)

來源

2017-08-22 inconsistent_coder

在問題中發佈完整的[堆棧跟蹤]（https://stackoverflow.com/q/3988788/2775450），並確定正在拋出它的代碼中的行。 –

請[編輯]你的問題，包括產生堆棧軌跡的[mcve]。 – trashgod

你意識到，除非你真的告訴我們，你是唯一知道哪一行是738行的人，對嗎？ –

我假設的錯誤造成的，因爲我刪除所選元素在ComboBox。

您可能需要重新檢查此假設。從example開始，我在明確和恢復ObservableList模型增加了以下按鈕。由於沒有錯誤發生，無論選擇何種情況，您都需要在別處尋找異常的原因。

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));

來源

2017-08-22 18:00:18 trashgod

非常感謝您的洞察力。奇怪的是，當我調用.clear（）時異常似乎發生。我已經編輯了更多信息和完整的堆棧跟蹤的主要帖子。 –

-1

我想你可能需要從你的組合框

comboBoxWorkflows .getItems().removeAll(workflowsComboList);

來源

2017-08-22 18:53:48

這相當於'workflowsComboList.removeAll（workflowsComboList）;'。充其量，這相當於OP已經發布的代碼 - 最糟糕的是它會拋出一個'ConcurrentModificationException'。 –

其實我之前曾嘗試過，正如James_D所說，我得到了同樣的例外。 –

刪除觀察的名單。如果你的組合框選擇綁定到另一個變量，請確保您處理一個空值。

來源

2017-08-22 20:36:17 benji2505

空指針異常清除一個JavaFX組合框

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