javafx 组合框<Integer>的 setValue(Integer) 方法导致 NullpointerException

Question

I have a ComboBox<Integer> where I want to set an initial selected value to. I also have a ChangeListener attached to the selectedItemProperty:

this.cbPlayerCount = new ComboBox<>(observableArrayList(2, 3, 4));
cbPlayerCount.getSelectionModel()
             .selectedItemProperty()
             .addListener(this::playerCountChanged);

cbPlayerCount.setValue(2);

The call of the setValue method triggers a chain of propertyChangeListeners (mine not included) and finally throws a NullpointerException . The signature of my listener method looks like this:

private void playerCountChanged(ObservableValue<?> val, int old, int newVal)

However the code of it is never invoked. The stacktrace looks like this:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
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 javafx.scene.control.SingleSelectionModel.clearAndSelect(SingleSelectionModel.java:103)
at javafx.scene.control.ComboBox.lambda$new$152(ComboBox.java:262)
at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:182)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:81)
at javafx.beans.property.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 de.dk.bm.menu.view.MenuView.<init>(MenuView.java:33)
...
at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
at java.lang.Thread.run(Thread.java:745)

I am using a jdk1.8.0_92. There is no error message or anything, just the Exception. I tried commenting the code that adds the ChangeListener, even though that code is never invoked. Without the listener attached, the Exception doesn't appear. But I still don't know why it is thrown when the listener is added. I don't want to debug the framework code to look for the mistake that leads to this issue. Why is this Exception thrown? Is it a bug in the javafx framework or did I just use it wrong?

Answer 1

The mistake was that I used the int type for the parameters of my playerCountChanged(Observable<?>, int, int) method.
So the first time a value is selected (by the call of the setValue method), there was no previous selected value, so the value passed as the parameter int old is null . Because I used int instead of Integer java tries to autobox the Integer value into an int value, which cannot be done with null .
So if I use Integer instead, the NullPointerException is thrown in my own code where I can fix it.