When to use JavaFX properties setter and getter, instead of using the property directly

Question

I am familiar with Java, but just starting to learn JavaFX, and specifically learn about JavaFX properties. I understand the basic design pattern as shown in the following

Answer 1

The JavaFX property pattern is designed to extend the old, standard JavaBean pattern. So in your example, according to the JavaBean convention, you have a (read-write) property of type double called amount. This is determined by the two methods

public double getAmount() ;
public void setAmount(double amount);

The JavaBean pattern allows some limited "observability" via "bound properties", in which beans support registering a PropertyChangeListener. A UI toolkit often has need to observe properties and respond to changes. For example, it makes sense for a Label to have a text property. If the text property changes, the Label needs to be notified, so that it knows to repaint itself. At first glance, using JavaBeans with bound properties would be a way to do this. However, using this mechanism in a UI toolkit produces performance issues, because there is no way to have notifications that a value is no longer valid without computing it immediately. This means, for example, that layout would be recomputed on every individual change to a property.

What the JavaFX team apparently were aiming to do was define a pattern that

1. conformed to the standard JavaBean pattern, and
2. supported observable properties where invalidation could be tracked without recomputing dependent values every time the value changed ("lazily observable values")

So the JavaFX solution is to create properties which support both ChangeListeners, which are notified when a value changes, and InvalidationListeners, which are notified when a value is no longer valid. This means that, for example, a layout mechanism can track whether or not it is currently valid without forcing a recomputation when it becomes invalid. The layout will only recompute on an actual screen pulse (i.e. when the scene is rendered), and only if it is invalid.

(As a quick proof-of-concept, consider the following:

DoubleProperty width = new SimpleDoubleProperty(3);
DoubleProperty height = new SimpleDoubleProperty(4);
ObservableDoubleValue area = Bindings.createDoubleBinding(() -> {
    double a = width.get() * height.get();
    System.out.println("Computed area: "+a);
    return a ;
}, width, height);
System.out.println("Area is "+area.getValue());
width.set(2);
height.set(3);
System.out.println("Area is "+area.getValue());

Note here that the intermediate value, when width is 2 and height is still 4, is never computed.)

So values in JavaFX are represented by these observable Properties which support both invalidation listeners and change listeners, meaning they are basically "lazily observable". Exposing the property itself via a property accessor method (amountProperty() in your example) is enough to support this functionality.

Semantically, however, exposing a DoubleProperty means the bean has a value of type double. In order to maintain compatibility with the old JavaBean convention, this bean should advertise this fact by exposing the corresponding get and set methods. Consequently, the JavaFX Property pattern requires both a "property accessor" (amountProperty()) as well as the standard JavaBean methods (getAmount() and setAmount(...)). This means that beans following the JavaFX pattern can be used anywhere the standard JavaBean pattern is used, for example in JPA.

Note that for the pattern to work correctly, it should always be true that amountProperty().get() == getAmount() and that amountProperty().set(x) has the same effect as setAmount(x). This is guaranteed (even if the bean class is subclassed) by making the get and set methods final, as in your example.

If you are invoking the methods yourself to retrieve or change the value of the property, it doesn't matter which you call, since they are guaranteed to have the same effect. Since the JavaFX Property pattern is an extension of the JavaBean pattern, there may be a very slight preference to call the get and set methods: in a sense accessing the value only needs the JavaBean functionality, not the full JavaFX property functionality, so it might make some semantic sense to only rely on that functionality. In practice, however, it makes no difference which you use.