An implementation of the ComboBoxBase abstract class for the most common form of ComboBox, where a popup list is shown to users providing them with a choice that they may select from. For more information around the general concepts and API of ComboBox, refer to the ComboBoxBase class documentation.

On top of ComboBoxBase, the ComboBox class introduces additional API. Most importantly, it adds an items property that works in much the same way as the ListView items property. In other words, it is the content of the items list that is displayed to users when they click on the ComboBox button.

The ComboBox exposes the valueProperty() from javafx.scene.control.ComboBoxBase, but there are some important points of the value property that need to be understood in relation to ComboBox. These include:

1. The value property is not constrained to items contained within the items list - it can be anything as long as it is a valid value of type T.
2. If the value property is set to a non-null object, and subsequently the items list is cleared, the value property is not nulled out.
3. Clearing the selection in the selection model does not null the value property - it remains the same as before.
4. It is valid for the selection model to have a selection set to a given index even if there is no items in the list (or less items in the list than the given index). Once the items list is further populated, such that the list contains enough items to have an item in the given index, both the selection model SelectionModel#selectedItemProperty() and value property will be updated to have this value. This is inconsistent with other controls that use a selection model, but done intentionally for ComboBox.

By default, when the popup list is showing, the maximum number of rows visible is 10, but this can be changed by modifying the visibleRowCount property. If the number of items in the ComboBox is less than the value of visibleRowCount, then the items size will be used instead so that the popup list is not exceedingly long.

As with ListView, it is possible to modify the selection model that is used, although this is likely to be rarely changed. This is because the ComboBox enforces the need for a javafx.scene.control.SingleSelectionModel instance, and it is not likely that there is much need for alternate implementations. Nonetheless, the option is there should use cases be found for switching the selection model.

As the ComboBox internally renders content with a ListView, API exists in the ComboBox class to allow for a custom cell factory to be set. For more information on cell factories, refer to the Cell and ListCell classes. It is important to note that if a cell factory is set on a ComboBox, cells will only be used in the ListView that shows when the ComboBox is clicked. If you also want to customize the rendering of the 'button' area of the ComboBox, you can set a custom ListCell instance in the button cell property. One way of doing this is with the following code (note the use of setButtonCell:

 Callback<ListView<String>, ListCell<String>> cellFactory = ...;
 ComboBox comboBox = new ComboBox();
 comboBox.setItems(items);
 comboBox.setButtonCell(cellFactory.call(null));
 comboBox.setCellFactory(cellFactory);

Because a ComboBox can be editable, and the default means of allowing user input is via a TextField, a string converter property is provided to allow for developers to specify how to translate a users string into an object of type T, such that the value property may contain it. By default the converter simply returns the String input as the user typed it, which therefore assumes that the type of the editable ComboBox is String. If a different type is specified and the ComboBox is to be editable, it is necessary to specify a custom StringConverter.

Warning: Nodes should not be inserted directly into the ComboBox items list

ComboBox allows for the items list to contain elements of any type, including Node instances. Putting nodes into the items list is strongly discouraged, as it can lead to unexpected results. This is because the default cell factory simply inserts Node items directly into the cell, including in the ComboBox 'button' area too. Because the scenegraph only allows for Nodes to be in one place at a time, this means that when an item is selected it becomes removed from the ComboBox list, and becomes visible in the button area. When selection changes the previously selected item returns to the list and the new selection is removed.

Important points to note:

• Avoid inserting Node instances directly into the ComboBox items list or its data model.
• The recommended approach is to put the relevant information into the items list, and provide a custom cell factory to create the nodes for a given cell and update them on demand using the data stored in the item for that cell.
• Avoid creating new Nodes in the updateItem method of a custom cell factory.

The following minimal example shows how to create a custom cell factory for ComboBox containing Nodes:

 ComboBox<Color> cmb = new ComboBox<>();
 cmb.getItems().addAll(
     Color.RED,
     Color.GREEN,
     Color.BLUE);

 cmb.setCellFactory(p -> {
     return new ListCell<>() {
         private final Rectangle rectangle;
         {
             setContentDisplay(ContentDisplay.GRAPHIC_ONLY);
             rectangle = new Rectangle(10, 10);
         }

         @Override protected void updateItem(Color item, boolean empty) {
             super.updateItem(item, empty);

             if (item == null || empty) {
                 setGraphic(null);
             } else {
                 rectangle.setFill(item);
                 setGraphic(rectangle);
             }
         }
     };
 });

This example has an anonymous custom ListCell class in the custom cell factory. Note that the Rectangle (Node) object needs to be created in the instance initialization block or the constructor of the custom ListCell class and updated/used in its updateItem method.

Admittedly the above approach is far more verbose, but it offers the required functionality without encountering the scenegraph constraints.