In an attempt to make some code "cheaper" I decided it could be a good idea to redundantly store the index next to the value it pointed to. Since this could save a fairly costly "getIndex" call. So I ended up with two properties; one public value and one private index. Basically both denote the same value, so I needed to add sychronisation code:

        // react to changes of the value
        this.valueObjectProperty.addListener(new ChangeListener<T>()
        {
            @Override
            public void changed(ObservableValue<? extends T> property, T oldValue, T newValue)
            {
                // get the value of the new index
                Object lIdx = getDataProvider().getIndex(newValue);
                if (lIdx == null) throw new IllegalArgumentException("Value does not exist " + newValue);

                // set the value
                indexObjectProperty.setValue(lIdx);
            }
        });

        // react to changes of the index
        this.indexObjectProperty.addListener(new ChangeListener<Object>()
        {
            @Override
            public void changed(ObservableValue<? extends Object> property, Object oldIndex, Object newIndex)
            {
                // get the value of the new index
                T lValue = (T)getDataProvider().getValue(newIndex);

                // set the value
                valueObjectProperty.setValue(lValue);
            }
        });


This code results in an endless loop:
Exception in Application start method
Exception in thread "main" java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.access$000(Unknown Source)
    at com.sun.javafx.application.LauncherImpl$1.run(Unknown Source)
    at java.lang.Thread.run(Thread.java:662)
Caused by: java.lang.StackOverflowError
    at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(Unknown Source)
    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(Unknown Source)
    at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(Unknown Source)
    at javafx.beans.property.ObjectPropertyBase.markInvalid(Unknown Source)
    at javafx.beans.property.ObjectPropertyBase.set(Unknown Source)
    at javafx.beans.property.ObjectProperty.setValue(Unknown Source)
    at jfxtras.scene.control.SpinnerX$1.changed(SpinnerX.java:147)
    at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(Unknown Source)
    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(Unknown Source)
    at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(Unknown Source)
    at javafx.beans.property.ObjectPropertyBase.markInvalid(Unknown Source)
    at javafx.beans.property.ObjectPropertyBase.set(Unknown Source)
    at javafx.beans.property.ObjectProperty.setValue(Unknown Source)
    at jfxtras.scene.control.SpinnerX$2.changed(SpinnerX.java:167)
    at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(Unknown Source)
    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(Unknown Source)
    at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(Unknown Source)
    at javafx.beans.property.ObjectPropertyBase.markInvalid(Unknown Source)
...


To solve this, I have to explicitly check if the value changed before the setValue (see "equals" below). Isn't Property suppose to do this?

        // react to changes of the value
        this.valueObjectProperty.addListener(new ChangeListener<T>()
        {
            @Override
            public void changed(ObservableValue<? extends T> property, T oldValue, T newValue)
            {
                // get the value of the new index
                Object lIdx = getDataProvider().getIndex(newValue);
                if (lIdx == null) throw new IllegalArgumentException("Value does not exist " + newValue);

                // set the value
                if (SpinnerX.equals(indexObjectProperty.getValue(), lIdx) == false)
                {
                    indexObjectProperty.setValue(lIdx);
                }
            }
        });

        // react to changes of the index
        this.indexObjectProperty.addListener(new ChangeListener<Object>()
        {
            @Override
            public void changed(ObservableValue<? extends Object> property, Object oldIndex, Object newIndex)
            {
                // get the value of the new index
                T lValue = (T)getDataProvider().getValue(newIndex);

                // set the value
                if (SpinnerX.equals(valueObjectProperty.getValue(), lValue) == false)
                {
                    valueObjectProperty.setValue(lValue);
                }
            }
        });