Combo boxes (drop-down lists)
Like a group of radio buttons, a drop-down list is a way to force the user to select only one element from a group of possibilities. However, it’s a more compact way to accomplish this, and it’s easier to change the elements of the list without surprising the user. (You can change radio buttons dynamically, but that tends to be visibly jarring).
By default, JComboBox box is not like the combo box in Windows, which lets you select from a list or type in your own selection. To produce this behavior you must call setEditable( ). With a JComboBox box, you choose one and only one element from the list. In the following example, the JComboBox box starts with a certain number of entries, and then new entries are added to the box when a button is pressed.
//: c14:ComboBoxes.java
// Using drop-down lists.
// <applet code=ComboBoxes width=200 height=125></applet>
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import com.bruceeckel.swing.*;
public class ComboBoxes extends JApplet {
private String[] description = {
"Ebullient", "Obtuse", "Recalcitrant", "Brilliant",
"Somnescent", "Timorous", "Florid", "Putrescent"
};
private JTextField t = new JTextField(15);
private JComboBox c = new JComboBox();
private JButton b = new JButton("Add items");
private int count = 0;
public void init() {
for(int i = 0; i < 4; i++)
c.addItem(description[count++]);
t.setEditable(false);
b.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
if(count < description.length)
c.addItem(description[count++]);
}
});
c.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
t.setText("index: "+ c.getSelectedIndex() + " " +
((JComboBox)e.getSource()).getSelectedItem());
}
});
Container cp = getContentPane();
cp.setLayout(new FlowLayout());
cp.add(t);
cp.add(c);
cp.add(b);
}
public static void main(String[] args) {
Console.run(new ComboBoxes(), 200, 125);
}
} ///:~
The JTextField displays the “selected index,” which is the sequence number of the currently selected element, as well as the text of the selected item in the combo box.
