Purpose of specialized BidirectionalBinding classes for primitive property types

[Michael Strauß]

The com.sun.javafx.binding.BidirectionalBinding class contains specialized
implementations for primitive types:
1. BidirectionalDoubleBinding
2. BidirectionalFloatBinding
3. BidirectionalIntegerBinding
4. BidirectionalLongBinding
5. BidirectionalBooleanBinding

These private implementation classes are selected by the static
BidirectionalBinding.bind(Property<T>, Property<T>) method iff the runtime
type of both provided property objects corresponds to the respective
primitive property implementation (DoubleProperty, FloatProperty, etc.).
In all other cases, the generic TypedGenericBidirectionalBinding
implementation is selected.

The only meaningful difference between the specialized classes and
TypedGenericBidirectionalBinding seems to be an attempt to avoid boxing
conversions by replacing the generic
    property1.setValue(newValue)
with variations of the following code:
    property1.set(newValue.doubleValue())

However, if I'm not mistaken, this doesn't prevent any boxing conversion,
thus rendering all of the specialized binding classes useless:

1. All of these classes implement the ChangeListener interface, which
provides the changed(ObservableValue<T>, T oldValue, T newValue) method.
2. Calling this method will box both 'oldValue' and 'newValue' parameters
if the values come from a primitive property type like DoubleProperty,
FloatProperty, etc.
3. Since both parameters are already boxed, the attempt to use
'set(newValue.doubleValue())' yields no advantage compared to just calling
'setValue(newValue)'.

If the purpose was indeed to prevent boxing conversions, these classes
would need to implement InvalidationListener, not ChangeListener.