Question

3.1 - In this exercise, you will see how to offer a user a selection among multiple choices. You can place all choices into a combo box using the following code:

/** File HelloViewer.java */

import javax.swing.JFrame;

public class HelloViewer

{

   public static void main(String[] args)

   {

      JFrame frame = new HelloFrame();

      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

      frame.setTitle("HelloViewer");

      frame.setVisible(true);

   }

}

----------------------------

/** File HelloFrame.java */

import javax.swing.JFrame;

import javax.swing.JLabel;

import java.awt.BorderLayout;

import java.awt.Font;

public class HelloFrame extends JFrame

{   

   private String message;

   private JLabel label;

   private static int FRAME_WIDTH = 300;

   private static int FRAME_HEIGHT = 300;

   private static int DEFAULT_SIZE = 20;

   public HelloFrame()

   {

      message = "Hello, World!";

      label = new JLabel(message);

      label.setFont(new Font("Serif", Font.PLAIN, DEFAULT_SIZE));

      add(label, BorderLayout.CENTER);

      setSize(FRAME_WIDTH, FRAME_HEIGHT);

   }

}

To build such a combo box, you add items to it:

final JComboBox comboBox = new JComboBox(); 

comboBox.addItem("Small"); 

comboBox.addItem("Medium"); 

comboBox.addItem("Large"); 

comboBox.addItem("Extra Large"); 

Then place the combo box in the south end of the frame.

add(comboBox, BorderLayout.SOUTH); 

Add a createSouthPanel method and paste the lines indicated above inside the method.

3.2 - The border layout grows all items to their maximum size. To avoid this, enclose the combo box inside a panel, even though it is a single item.

JPanel southPanel = new JPanel(); 

southPanel.add(comboBox); 

add(southPanel, BorderLayout.SOUTH);

Add the combo box to the frame of the HelloViewer program.

3.3 - To activate the combo box, you need to attach an action listener. The actionPerformed method needs to determine which item the user selected.

class ComboListener implements ActionListener 

{

   public void actionPerformed(ActionEvent event) 

   {

      String item = (String) comboBox.getSelectedItem(); 

      . . . 

   }

}

You need to cast the return value of the getSelectedItem method to a String because it is possible to put other objects, such as icons, into a combo box.

Now you simply set the correct font size, depending on the item string.

int messageSize = DEFAULT_SIZE; 

if (item.equals("Small")) { messageSize = SMALL_SIZE; } 

else if (item.equals("Medium")) { messageSize = MEDIUM_SIZE; } 

else if (item.equals("Large")) { messageSize = LARGE_SIZE; } 

else if (item.equals("Extra Large")) { messageSize = EXTRA_LARGE_SIZE; }

label.setFont(new Font("Serif", Font.PLAIN, messageSize));

label.repaint();

Define the size constants as follows:

public static final int SMALL_SIZE = 12; 

public static final int MEDIUM_SIZE = 18; 

public static final int LARGE_SIZE = 24; 

public static final int EXTRA_LARGE_SIZE = 36; 

When you select an item from the combo box, the message should be displayed in the appropriate size.

What is the complete code now?