Cách Thiết kế UI bằng JAVA SWING !

Chapter 9. Combo Boxes
In this chapter:
JComboBox
Basic JComboBox example
Custom model and renderer
Combo box with memory
Custom editing

9.1 JCombobox
class javax.swing.JComboBox
This class represents a basic GUI component which consists of two parts:
A popup menu (an implementation of javax.swing.plaf.basic.ComboPopup). By default this is a JPopupMenu sub-class (javax.swing.plaf.basic.BasicComboPopup) containing a JList in a JScrollPane.
A button acting as a container for an editor or renderer component, and an arrow button used to display the popup menu.

The JList uses a ListSelectionModel (see chapter 10) allowing SINGLE_SELECTION only. Apart from this, JComboBox directly uses only one model, a ComboBoxModel, which manages data in its JList.

A number of constructors are available to build a JComboBox. The default constructor can be used to create a combo box with an empty list, or we can pass data to a constructor as a one-dimensional array, a Vector, or as an implementation of the ComboBoxModel interface (see below). The last variant allows maximum control over the properties and appearance of a JComboBox, as we will see.

As other complex Swing components, JComboBox allows a customizable renderer for displaying each item in its drop-down list (by default a JLabel sub-class implementation of ListCellRenderer), and a customizable editor to be used as the combo box’s data entry component (by default an instance of ComboBoxEditor which uses a JTextField). We can use the existing default implementations of ListCellRenderer and ComboBoxEditor, or we can create our own according to our particular needs (which we will see later in ths chapter). Note that unless we use a custom renderer, the default renderer will display each element as a String defined by that object’s toString() method (the only exceptions to this are Icon implementations which will be renderered as they would be in any JLabel). Also note that a renderer returns a Component, but that component is not interactive and is only used for display purposes (i.e. it acts as a “rubber stamp”API). For instance, if a JCheckBox is used as a renderer we will not be able to check and uncheck it. Editors, however, are fully interactive.

Similar to JList (next chapter), this class uses to deliver information about changes in the state of its drop-down list’s model. and are fired when the current selection changes (from any source--programmatic or direct user input). Correspondingly, we can attach and to receive these events.

The drop-down list of a JComboBox is a popup menu containing a JList (this is actually defined in the UI delegate, not the component itself) and can be programmatically displayed using the showPopup() and hidePopup() methods. As any other Swing popup menu (which we will discuss in chapter 12), it can be displayed as either heavyweight or lightweight. JComboBox provides the setLightWeightPopupEnabled() method allowing us to choose between these modes.

JComboBox also defines an inner interface called KeySelectionManager that declares one method, selectionForKey(char aKey, ComboBoxModel aModelwhich should be overriden to return the index of the list element to select when the list is visible (popup is showing) and the given keyboard character is pressed.

The JComboBox UI delegate represents JComboBox graphically by a container with a button which encapsulates an arrow button and either a renderer displaying the currently selected item, or an editor allowing changes to be made to the currently selected item. The arrow button is displayed on the right of the renderer/editor and will show the popup menu containing the drop-down list when clicked.

Note: Because of the JComboBox UI delegate construction, setting the border of a JComboBox does not have the expected effect. Try this and you will see that the container containing the main JComboBox button gets the assigned border, when in fact we want that button to recieve the border. There is no easy way to set the border of this button without customizing the UI delegate, and we hope to see this limitation disappear in a future version.

When a JComboBox is editable (which it is not by default) the editor component will allow modification of the currenly selected item. The default editor will appear as a JTextField accepting input. This text field has an ActionListener attached that will accept an edit and change the selected item accoringly when/if the Enter key is pressed. If the focus changes while editing, all editing will be cancelled and a change will not be made to the selected item.

JComboBox can be made editable with its setEditable() method, and we can specify a custom ComboBoxEditor with setEditor() method.. Setting the editable property to true causes the UI delegate to replace the renderer component in the button to the specified editor component. Similarly, setting this property to false causes the editor in the button to be replaced by a renderer.

The cell renderer used for a JComboBox can be assigned/retrieved with the setRenderergetRenderer() methods. Calls to these methods