# Characteristics of Bindings

Bindings are assembled from one or more sources, usually called their dependencies. A binding observes its dependencies for changes and updates its own value according to changes in the dependencies.

Almost all bindings defined in this library require implementations of {@link javafx.beans.Observable} for their dependencies. There are two types of implementations already provided, the properties in the package {@link javafx.beans.property} and the observable collections ({@link javafx.collections.ObservableList} and {@link javafx.collections.ObservableMap}). Bindings also implement {@code Observable} and can again serve as sources for other bindings allowing to construct very complex bindings from simple ones.

Bindings in our implementation are always calculated lazily. That means, if a dependency changes, the result of a binding is not immediately recalculated, but it is marked as invalid. Next time the value of an invalid binding is requested, it is recalculated.

# High Level API and Low Level API

The Binding API is roughly divided in two parts, the High Level Binding API and the Low Level Binding API. The High Level Binding API allows to construct simple bindings in an easy to use fashion. Defining a binding with the High Level API should be straightforward, especially when used in an IDE that provides code completion. Unfortunately it has its limitation and at that point the Low Level API comes into play. Experienced Java developers can use the Low Level API to define bindings, if the functionality of the High Level API is not sufficient or to improve the performance. The main goals of the Low Level API are fast execution and small memory footprint.

Following is an example of how both APIs can be used. Assuming we have four instances of {@link javafx.beans.property.DoubleProperty} {@code a}, {@code b}, {@code c} , and {@code d}, we can define a binding that calculates {@code a*b + c*d} with the High Level API for example like this:

{@code NumberBinding result = Bindings.add (a.multiply(b), c.multiply(d)); }

Defining the same binding using the Low Level API could be done like this:

    DoubleBinding foo = new DoubleBinding() {

        {
            super.bind(a, b, c, d);
        }

        @Override
        protected double computeValue() {
            return a.getValue() * b.getValue() + c.getValue() * d.getValue();
        }
    };