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 example from Oracle:
package propertydemo;
import javafx.beans.property.DoubleProperty;
import javafx.beans.property.SimpleDoubleProperty;
class Bill {
// Define a variable to store the property
private DoubleProperty amountDue = new SimpleDoubleProperty();
// Define a getter for the property's value
public final double getAmountDue(){return amountDue.get();}
// Define a setter for the property's value
public final void setAmountDue(double value){amountDue.set(value);}
// Define a getter for the property itself
public DoubleProperty amountDueProperty() {return amountDue;}
}
What I don't understand is when/why I would use the getter and setter methods, instead of using the Property directly?
What I was thinking is that you may want some custom code in the getter and/or setter that may do some pre or post manipulation/validation of the data, but if you create a custom getter and/or setter, you would get different results then, depending on whether you use the getter/setter or property directly, which to me, seems dangerous.
If the getter/setter simply call the Property's get and set methods, then why even have them?
Any insights into this would be appreciated.
Getters and setters are used to protect your data, particularly when creating classes. For each instance variable, a getter method returns its value while a setter method sets or updates its value. Given this, getters and setters are also known as accessors and mutators, respectively.
1) getter and setter method gives you centralized control on how a the particular field is initialized and provided to the client which makes the validation and debugging much easier. you can simply put breakpoints or print statement to see which thread are accessing and what values are going out.
The getter and setter method gives you centralized control of how a certain field is initialized and provided to the client, which makes it much easier to verify and debug. To see which thread is accessing and what values are going out, you can easily place breakpoints or a print statement.
Getters and Setters play an important role in retrieving and updating the value of a variable outside the encapsulating class. A setter updates the value of a variable, while a getter reads the value of a variable.
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
So the JavaFX solution is to create properties which support both ChangeListener
s, which are notified when a value changes, and InvalidationListener
s, 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.
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