Choice Controls - AWT Controls

Choice Controls

The Choice class is used to create a pop-up list of items from which the user may choose. Thus, a Choice control is a form of menu. When inactive, a Choice component takes up only enough space to show the currently selected item. When the user clicks on it, the whole list of choices pops up, and a new selection can be made. Each item in the list is a string that appears as a left-justified label in the order it is added to the Choice object. Choice defines only the default constructor, which creates an empty list.

To add a selection to the list, call add( ). It has this general form: void add(String name)

Here, name is the name of the item being added. Items are added to the list in the order in which calls to add( ) occur.

To determine which item is currently selected, you may call either getSelectedItem( ) or getSelectedIndex( ). These methods are shown here:

String getSelectedItem( ) 

int getSelectedIndex( )

The getSelectedItem( ) method returns a string containing the name of the item. getSelectedIndex( ) returns the index of the item. The first item is at index 0. By default, the first item added to the list is selected.

To obtain the number of items in the list, call getItemCount( ). You can set the currently selected item using the select( ) method with either a zero-based integer index or a string that will match a name in the list. These methods are shown here:

int getItemCount( ) void select(int index) void select(String name)

Given an index, you can obtain the name associated with the item at that index by calling getItem( ), which has this general form:

String getItem(int index)

Here, index specifies the index of the desired item.

Handling Choice Lists

Each time a choice is selected, an item event is generated. This is sent to any listeners that previously registered an interest in receiving item event notifications from that component. Each listener implements the ItemListener interface. That interface defines the itemStateChanged( ) method. An ItemEvent object is supplied as the argument to this method.

Here is an example that creates two Choice menus. One selects the operating system. The other selects the browser.

// Demonstrate Choice lists.

import java.awt.*;

import java.awt.event.*;

import java.applet.*; /*

<applet code="ChoiceDemo" width=300 height=180> </applet>

*/

public class ChoiceDemo extends Applet implements ItemListener { Choice os, browser;

String msg = "";

public void init() { os = new Choice();

browser = new Choice();

     //add items to os list

     os.add("Windows"); os.add("Android"); os.add("Solaris"); os.add("Mac OS");

     //add items to browser list

     browser.add("Internet Explorer"); browser.add("Firefox"); browser.add("Chrome");

     //add choice lists to window

     add(os);

add(browser);

     //register to receive item events

     os.addItemListener(this); browser.addItemListener(this);

}

public void itemStateChanged(ItemEvent ie) { repaint();

}

// Display current selections.

public void paint(Graphics g) {

msg = "Current OS: ";

msg += os.getSelectedItem(); g.drawString(msg, 6, 120); msg = "Current Browser: ";

msg += browser.getSelectedItem(); g.drawString(msg, 6, 140);

}

Sample output is shown in Figure 26-4.

Figure 26-4   Sample output from the ChoiceDemo applet