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Trail: Creating a GUI with JFC/Swing
Lesson: Using Swing Components

How to Use Combo Boxes

A JComboBox(in the API reference documentation) comes in two very different forms: uneditable and editable.

By default, a combo box is uneditable. A uneditable combo box looks like a button until the user interacts with it. When the user presses or clicks it, the combo box displays a menu of items to choose from. Use an uneditable combo box to display one-of-many choices when space is limited, when the number of choices is large, or when the menu items are computed at run-time. Other components that display one-of-many choices are lists and groups of radio buttons.

An editable JComboBox looks like a text field with a small button abutting it. The user can type a value in the text field or click the button to choose a value from a menu. An editable combo box saves data-entry time by providing shortcuts to commonly entered values.

Because editable and uneditable combo boxes are so different, this section treats them separately. This section covers these topics:

Using an Uneditable Combo Box
The application shown here uses an uneditable combo box for choosing a pet picture:

An uneditable combo box

Try this: 
  1. Compile and run the program: ComboBoxDemo.java(in a .java source file). You will also need five image files.
    See Getting Started with Swing if you need help compiling or running this application.
  2. Choose a pet name from the combo box to view its picture.
  3. How to Use Radio Buttons provides a version of this program, RadioButtonDemo, that uses a group of radio buttons instead of a combo box. Compile and run that program. Compare the source code, operation, and UI of the two programs.
The following code, taken from ComboBoxDemo.java(in a .java source file), creates an uneditable combo box and sets it up: 
String[] petStrings = { "Bird", "Cat", "Dog", "Rabbit", "Pig" };

// Create the combo box, select item at index 4.
// Indices start at 0, so 4 specifies the pig.
JComboBox petList = new JComboBox(petStrings);
petList.setSelectedIndex(4);
...
The code initializes the combo box with an array of strings. You can also put icons in a combo box. To put other types of objects in a combo box, or to customize how the items in a combo box look, you need to write a custom renderer. An editable combo box would also need a custom editor. Refer to Providing a Custom Renderer for information and an example.

Although not shown above, the program registers an action listener on the combo box. To see the code and learn about other types of listeners supported by combo box, refer to Handling Events on a Combo Box.

No matter which constructor you use, a combo box uses a combo box model to contain and manage the items in its menu. When you initialize a combo box with an array or a vector, the combo box creates a default model object for you. As with other Swing components, you can customize a combo box in part by implementing a custom model -- an object that implements the ComboBoxModel(in the API reference documentation) interface.

Note:  Be careful when implementing a custom model for a combo box. The JComboBox methods that change the items in the combo box's menu, such as insertItemAt, work only if the data model implements the MutableComboBoxModel(in the API reference documentation) interface (a subinterface of ComboBoxModel). Refer to the API tables to see which methods are affected.

Handling Events on a Combo Box
Here's the code from ComboBoxDemo that registers and implements an action listener on the combo box: 
petList.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        JComboBox cb = (JComboBox)e.getSource();
        String petName = (String)cb.getSelectedItem();
        picture.setIcon(new ImageIcon("images/" +
                                      petName + ".gif"));
    }
});
This action listener gets the newly selected item from the combo box, uses it to compute the name of an image file, and updates a label to display the image. The combo box fires an action event when the user selects an item from the combo box's menu. See How to Write an Action Listener(in the Creating a GUI with JFC/Swing trail), for general information about implementing action listeners.

Combo boxes also generate item events, which are fired when any of the items' selection state changes. Only one item at a time can be selected in a combo box, so when the user makes a new selection the previously selected item becomes unselected. Thus two item events are fired each time the user selects a different item from the menu. If the user chooses the same item, no item events are fired. Use addItemListener to register an item listener on a combo box. How to Write an Item Listener(in the Creating a GUI with JFC/Swing trail) gives general information about implementing item listeners.

Although JComboBox inherits methods to register listeners for low-level events -- focus, key, and mouse events, for example -- we recommend that you don't listen for low-level events on a combo box. Here's why: A combo box is a compound component -- it is comprised of two or more other components. The combo box itself fires high-level events such as action events. Its subcomponents fire low-level events such as mouse, key, and focus events. The low-level events and the subcomponent that fires them are look-and-feel-dependent. To avoid writing look-and-feel-dependent code, you should listen only for high-level events on a compound component such as a combo box. For information about events, including a discussion about high- and low-level events, refer to Writing Event Listeners(in the Creating a GUI with JFC/Swing trail).

Using an Editable Combo Box
Here's a picture of a demo application, provided by our coworker Dale Green, that uses an editable combo box to enter a pattern with which to format dates.

An editable combo box

Try this: 
  1. Compile and run the example: ComboBoxDemo2.java(in a .java source file).
    See Getting Started with Swing if you need help compiling or running this application.
  2. Enter a new pattern by choosing one from the combo box's menu. The program reformats the current date and time.
  3. Enter a new pattern by typing one in and pressing Return. Again the program reformats the current date and time.

The following code, taken from ComboBoxDemo2.java(in a .java source file), creates and sets up the combo box: 

String[] patternExamples = {
	 "dd MMMMM yyyy",
	 "dd.MM.yy",
	 "MM/dd/yy",
	 "yyyy.MM.dd G 'at' hh:mm:ss z",
	 "EEE, MMM d, ''yy",
	 "h:mm a",
	 "H:mm:ss:SSS",
	 "K:mm a,z",
	 "yyyy.MMMMM.dd GGG hh:mm aaa"
};
. . .
JComboBox patternList = new JComboBox(patternExamples);
patternList.setEditable(true);
patternList.addActionListener(...);
. . .
This code is very similar to the previous example, but warrants a few words of explanation. The bold line of code explicitly turns on editing to allow the user to type values in. This is necessary because, by default, a combo box is not editable. This particular example allows editing on the combo box because its menu does not provide all possible date formatting patterns, just shortcuts to frequently used patterns.

An editable combo box fires an action event when the user chooses an item from the menu and when the user types Return. Note that the menu remains unchanged when the user enters a value into the combo box. If you want, you can easily write an action listener that adds a new item to the combo box's menu each time the user types in a unique value.

See Internationalization(in the Creating a GUI with JFC/Swing trail) to learn more about formatting dates and other types of data.

Providing a Custom Renderer

A combo box uses a renderer to display each item in its menu. If the combo box is uneditable, it also uses the renderer to display the currently selected item. An editable combo box, on the other hand, uses an editor to display the selected item. A renderer for a combo box must implement the ListCellRenderer(in the API reference documentation) interface. A combo box's editor must implement ComboBoxEditor(in the API reference documentation). This section shows how to provide a custom renderer for an uneditable combo box.

The default renderer knows how to render strings and icons. If you put other objects in a combo box, the default renderer calls the toString method to provide a string to display. You can customize the way a combo box renders itself and its items by implementing your own ListCellRenderer.

Here's a picture of an application that uses a combo box with a custom renderer:

A combo box with a custom renderer

You can find the full source code for this example in CustomComboBoxDemo.java(in a .java source file). This example needs the five pet image files.

The following statements from the example create an instance of ComboBoxRenderer (a custom class) and set up the instance as the combo box's renderer: 

JComboBox petList = new JComboBox(images);
ComboBoxRenderer renderer = new ComboBoxRenderer();
renderer.setPreferredSize(new Dimension(200, 130));
petList.setRenderer(renderer);
petList.setMaximumRowCount(3);
To see how the default renderer renders icons, just comment out the call to setRenderer, and compile and run the program again.

The code also sets the combo box's maximum row count. This attribute controls the number of items visible when the menu is displayed. If the number of items in the combo box is larger than its maximum row count, the menu has a scroll bar. The icons are pretty big for a menu, so the code limits the number of rows to 3. Here's the complete implementation of ComboBoxRenderer, a renderer that puts an icon and text side-by-side: 

class ComboBoxRenderer extends JLabel
                       implements ListCellRenderer {
    public ComboBoxRenderer() {
        setOpaque(true);
        setHorizontalAlignment(CENTER);
        setVerticalAlignment(CENTER);
    }
    public Component getListCellRendererComponent(
        JList list,
        Object value,
        int index,
        boolean isSelected,
        boolean cellHasFocus)
    {
        if (isSelected) {
            setBackground(list.getSelectionBackground());
            setForeground(list.getSelectionForeground());
        } else {
            setBackground(list.getBackground());
            setForeground(list.getForeground());
        }

        ImageIcon icon = (ImageIcon)value;
        setText(icon.getDescription());
        setIcon(icon);
        return this;
    }
}
As a ListCellRenderer, ComboBoxRenderer implements a method called getListCellRendererComponent, which returns a component whose paintComponent method is used to display the combo box and each of its items. The easiest way to display an image and an icon is to use a label. So ComboBoxRenderer is a subclass of label and returns itself. The implementation of getListCellRendererComponent configures the renderer to display the currently selected icon and its description.

These arguments are passed to getListCellRendererComponent:

Note that combo boxes and lists use the same type of renderer -- ListCellRenderer. You can save yourself some time by sharing renderers between combo boxes and lists, if it makes sense for your program.

The Combo Box API

The following tables list the commonly used JComboBox constructors and methods. Other methods you are most likely to invoke on a JComboBox object are those it inherits from its superclasses, such as setPreferredSize. See The JComponent API for tables of commonly used inherited methods.

The API for using combo boxes falls into two categories:

Setting or Getting the Items in the Combo Boxes's Menu
Method Purpose
JComboBox()
JComboBox(ComboBoxModel)
JComboBox(Object[])
JComboBox(Vector) 		Create a combo box with the specified items in its menu. A combo box created with the default constructor has no items in the menu initially. Each of the other constructors initializes the menu from its argument: a model object, an array of objects, or a Vector of objects.
void addItem(Object)
void insertItemAt(Object, int) 		Add or insert the specified object into the combo box's menu. The insert method places the specified object at the specified index, thus inserting it before the object currently at that index. These methods require that the combo box's data model be an instance of MutableComboBoxModel.
Object getItemAt(int)
Object getSelectedItem() 		Get an item from the combo box's menu.
void removeAllItems()
void removeItemAt(int)
void removeItem(Object) 		Remove one or more items from the combo box's menu. These methods require that the combo box's data model be an instance of MutableComboBoxModel.
int getItemCount() Get the number of items in the combo box's menu.
void setModel(ComboBoxModel)
ComboBoxModel getModel() 		Set or get the data model that provides the items in the combo box's menu.
void setPrototypeDisplayValue(Object)
Object getPrototypeDisplayValue() 		Set or get the value used to determine the preferred size of the combo box. The use of a prototype object can improve performance. Introduced in 1.4.

Customizing the Combo Box's Operation
Method or Constructor Purpose
void addActionListener(ActionListener) Add an action listener to the combo box. The listener's actionPerformed method is called when the user selects an item from the combo box's menu or, in an editable combo box, when the user presses Return.
void addItemListener(ItemListener) Add an item listener to the combo box. The listener's itemStateChanged method is called when the selection state of any of the combo box's items change.
void setEditable(boolean)
boolean isEditable() 		Set or get whether the user can type in the combo box.
void setRenderer(ListCellRenderer)
ListCellRenderer getRenderer() 		Set or get the object responsible for painting the selected item in the combo box. The renderer is used only when the combo box is uneditable. If the combo box is editable, the editor is used to paint the selected item instead.
void setEditor(ComboBoxEditor)
ComboBoxEditor getEditor() 		Set or get the object responsible for painting and editing the selected item in the combo box. The editor is used only when the combo box is editable. If the combo box is uneditable, the renderer is used to paint the selected item instead.

Examples that Use Combo Boxes

This table shows the examples that use JComboBox and where those examples are described.

Example Where Described Notes
ComboBoxDemo This section Uses an uneditable combo box.
ComboBoxDemo2 This section Uses an editable combo box.
CustomComboBoxDemo This section Provides a custom renderer for a combo box.
TableRenderDemo How to Use Tables (Further Customizing Table Display and Event Handling) Shows how to use a combo box as a table cell editor.


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