Lists and Combo Boxes
JLists and JComboBoxes are a step up on
the evolutionary chain from JButtons and
JLabels. Lists let the user choose from a group of
alternatives. They can be configured to force the user to choose
a single selection or to allow multiple choices. Usually, only a
small group of choices are displayed at a time; a scrollbar lets
the user move to the choices that aren't visible. The user can
select an item by clicking on it. He or she can expand the
selection to a range of items by holding down Shift and clicking
on another item. To make discontinuous selections, the user can
hold down the Control key instead of the Shift key.
A combo box is a cross-breed between a text field and a list. It
displays a single line of text (possibly with an image) and a
downward pointing arrow at one side. If you click on the arrow,
the combo box opens up and displays a list of choices. You can
select a single choice by clicking on it. After a selection is
made, the combo box closes up; the list disappears and the new
selection is shown in the text field.
Like every other component in Swing, lists and combo boxes have
data models that are distinct from visual components. The list
also has a selection model that controls how selections
may be made on the list data.
Lists and combo boxes are similar because they have similar data
models. Each is simply an array of acceptable choices. This
similarity is reflected in Swing, of course: the type of a
JComboBox's data model is a subclass of the type
used for a JList's data model. The next example
demonstrates this relationship.
The following example creates a window with a combo box, a list,
and a button. The combo box and the list use the same data model.
When you press the button, the program writes out the current set
of selected items in the list. Figure 14-2 shows the example; the
code itself follows.
Figure 14-2. A combo box and a list using the same data model
/file: Lister.java
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Lister {
public static void main(String[] args) {
JFrame f = new JFrame("Lister v1.0");
f.setSize(200, 200);
f.setLocation(200, 200);
f.addWindowListener(new WindowAdapter( ) {
public void windowClosing(WindowEvent we) { System.exit(0); }
});
// create a combo box
String [] items = { "uno", "due", "tre", "quattro", "cinque",
"sei", "sette", "otto", "nove", "deici",
"undici", "dodici" };
JComboBox comboBox = new JComboBox(items);
comboBox.setEditable(true);
// create a list with the same data model
final JList list = new JList(comboBox.getModel( ));
// create a button; when it's pressed, print out
// the selection in the list
JButton button = new JButton("Per favore");
button.addActionListener(new ActionListener( ) {
public void actionPerformed(ActionEvent ae) {
Object[] selection = list.getSelectedValues( );
System.out.println("-----");
for (int i = 0; i < selection.length; i++)
System.out.println(selection[i]);
}
});
// put the controls the content pane
Container c = f.getContentPane( );
JPanel comboPanel = new JPanel( );
comboPanel.add(comboBox);
c.add(comboPanel, BorderLayout.NORTH);
c.add(new JScrollPane(list), BorderLayout.CENTER);
c.add(button, BorderLayout.SOUTH);
f.setVisible(true);
}
}
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