endless loop on property sync code

[Tom Eugelink]

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);
                 }
             }
         });