JavaFX Spinner empty text nullpointerexception

Question

I have an issue where an editable JavaFX 8 Spinner causes an uncaught NullPointerException if one clears the editor text and commits and then click

Answer 1

I had a rummage through the JDK source.

The NPE is thrown from if (newValue < getMin()) { in the listener lambda here:

javafx.scene.control.SpinnerValueFactory.java

    public IntegerSpinnerValueFactory(@NamedArg("min") int min,
                                      @NamedArg("max") int max,
                                      @NamedArg("initialValue") int initialValue,
                                      @NamedArg("amountToStepBy") int amountToStepBy) {
        setMin(min);
        setMax(max);
        setAmountToStepBy(amountToStepBy);
        setConverter(new IntegerStringConverter());

        valueProperty().addListener((o, oldValue, newValue) -> {
            // when the value is set, we need to react to ensure it is a
            // valid value (and if not, blow up appropriately)
            if (newValue < getMin()) { 
                setValue(getMin());
            } else if (newValue > getMax()) {
                setValue(getMax());
            }
        });
        setValue(initialValue >= min && initialValue <= max ? initialValue : min);
    }

presumably newValue is null and the auto unboxing of null throws NPE. As the input comes from the editor, I suspect the IntegerStringConverter which is the default converter.

Looking at the implementation here:

javafx.util.converter.IntegerStringConverter

public class IntegerStringConverter extends StringConverter {
    /** {@inheritDoc} */
    @Override public Integer fromString(String value) {
        // If the specified value is null or zero-length, return null
        if (value == null) {
            return null;
        }

        value = value.trim();

        if (value.length() < 1) {
            return null;
        }

        return Integer.valueOf(value);
    }

    /** {@inheritDoc} */
    @Override public String toString(Integer value) {
        // If the specified value is null, return a zero-length String
        if (value == null) {
            return "";
        }

        return (Integer.toString(((Integer)value).intValue()));
    }
}

We see that it will happily return null for the empty string, which is kind of reasonable given that there exists no valid value for the input.

Tracing up the call stack I find where the value is coming from:

javafx.scene.control.Spinner

public Spinner() {
    getStyleClass().add(DEFAULT_STYLE_CLASS);
    setAccessibleRole(AccessibleRole.SPINNER);

    getEditor().setOnAction(action -> {
        String text = getEditor().getText();
        SpinnerValueFactory valueFactory = getValueFactory();
        if (valueFactory != null) {
            StringConverter converter = valueFactory.getConverter();
            if (converter != null) {
                T value = converter.fromString(text);
                valueFactory.setValue(value);
            }
        }
    });

The value is set with the value obtained from the converter T value = converter.fromString(text); which presumably is null. At this point I believe that the spinner class should check that value is not null and if it is restore the previous value to the editor.

I am now fairly sure that this is a bug. Moreover I don't think that a work around with a converter that never returns null is going to work properly as it will only mask the problem and what value should be returned when the value cannot be converted?

Edit: Workaround

Replacing the onAction of the spinner editor to reject invalid input with a "return to valid" policy fixes the issue:

public static  void fixSpinner2(Spinner aSpinner) {
    aSpinner.getEditor().setOnAction(action -> {
        String text = aSpinner.getEditor().getText();
        SpinnerValueFactory factory = aSpinner.getValueFactory();
        if (factory != null) {
            StringConverter converter = factory.getConverter();
            if (converter != null) {
                T value = converter.fromString(text);
                if (null != value) {
                    factory.setValue(value);
                }
                else {
                    aSpinner.getEditor().setText(converter.toString(factory.getValue()));
                }
            }
        }
        action.consume();
    });
}

As opposed to a listener on the valueProperty this avoids triggering other listeners with invalid data. However this highlights another issue in the spinner class. While the above fixes the issue by returning to a valid value on pressing enter. Erasing the input without committing (pressing enter) and then pressing increment or decrement will cause the same NPE but with slightly different call stack.

Cause:

public void increment(int steps) {
    SpinnerValueFactory valueFactory = getValueFactory();
    if (valueFactory == null) {
        throw new IllegalStateException("Can't increment Spinner with a null SpinnerValueFactory");
    }
    commitEditorText();
    valueFactory.increment(steps);
}

Decrement is similar, both call into commitEditorText below:

private void commitEditorText() {
    if (!isEditable()) return;
    String text = getEditor().getText();
    SpinnerValueFactory valueFactory = getValueFactory();
    if (valueFactory != null) {
        StringConverter converter = valueFactory.getConverter();
        if (converter != null) {
            T value = converter.fromString(text);
            valueFactory.setValue(value);
        }
    }
}

Notice the copy-paste from the onAction in the constructor:

    getEditor().setOnAction(action -> {
        String text = getEditor().getText();
        SpinnerValueFactory valueFactory = getValueFactory();
        if (valueFactory != null) {
            StringConverter converter = valueFactory.getConverter();
            if (converter != null) {
                T value = converter.fromString(text);
                valueFactory.setValue(value);
            }
        }
    });

I believe that commitEditorText should be changed to trigger onAction on the editor instead like so:

private void commitEditorText() {
    if (!isEditable()) return;
    getEditor().getOnAction().handle(new ActionEvent(this, this));
}

then the behavior would be consistent and give the editor a chance to handle the input before it goes to the value factory.