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Java AWT

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9. Pick Me

Three AWT components let you present a list of choices to users: Choice, List, and Checkbox. All three components implement the ItemSelectable interface (  Java1.1). These components are comparable to selection mechanisms in modern GUIs so most readers will be able to learn them easily, but I'll point out some special enhancements that they provide.

Choice and List are similar; both offer a list of choices for the user to select. Choice provides a pull-down list that offers one selection at a time, whereas List is a scrollable list that allows a user to make one or multiple selections. From a design standpoint, which you choose depends at least partially on screen real estate; if you want the user to select from a large group of alternatives, Choice requires the least space, List requires somewhat more, while Checkbox requires the most. Choice is the only component in this group that does not allow multiple selections. A List allows multiple or single selection; because each Checkbox is a separate component, checkboxes inherently allow multiple selection. In order to create a list of mutually exclusive checkboxes, in which only one box can be selected at a time (commonly known as radio buttons), you can put several checkboxes together into a CheckboxGroup, which is discussed at the end of this chapter.

9.1 Choice

The Choice component provides pop-up/pull-down lists. It is the equivalent of Motif's OptionMenu or Windows MFC's ComboBox. (  Java 1.1 departs from the MFC world.) With the Choice component, you can provide a short list of choices to the user, while taking up the space of a single item on the screen. When the component is selected, the complete list of available choices appears on the screen. After the user has selected an option, the list is removed from the screen and the selected item is displayed. Selecting any item automatically deselects the previous selection.

Component Methods

Constructors

public Choice ()

There is only one constructor for Choice. When you call it, a new instance of Choice is created. The component is initially empty, with no items to select. Once you add some items using addItem() (version 1.0) or add() (version 1.1) and display the Choice on the screen, it will look something like the leftmost component in Figure 9.1. The center component shows what a Choice looks like when it is selected, while the one on the right shows what a Choice looks like before any items have been added to it.

Items

public int getItemCount () (New)
public int countItems () (Deprecated)

The getItemCount() method returns the number of selectable items in the Choice object. In Figure 9.1, getItemCount() would return 6.

countItems() is the Java 1.0 name for this method.

public String getItem (int index)

The getItem() method returns the text for the item at position index in the Choice. If index is invalid--either index < 0 or index >= getItemCount()--the getItem() method throws the ArrayIndexOutOfBoundsException run-time exception.

public synchronized void add (String item) (New)
public synchronized void addItem (String item) (Deprecated)

add() adds item to the list of available choices. If item is already an option in the Choice, this method adds it again. If item is null, add() throws the run-time exception NullPointerException. The first item added to a Choice becomes the initial (default) selection.

addItem() is the Java 1.0 name for this method.

public synchronized void insert (String item, int index) (New)

insert() adds item to the list of available choices at position index. An index of 0 adds the item at the beginning. An index larger than the number of choices adds the item at the end. If item is null, insert() throws the run-time exception NullPointerException. If index is negative, insert() throws the run-time exception IllegalArgumentException.

public synchronized void remove (String item) (New)

remove() removes item from the list of available choices. If item is present in Choice multiple times, a call to remove() removes the first instance. If item is null, remove() throws the run-time exception NullPointerException. If item is not found in the Choice, remove() throws the IllegalArgumentException run-time exception.

public synchronized void remove (int position) (New)

remove() removes the item at position from the list of available choices. If position is invalid--either position < 0 or position >= getItemCount()--remove() throws the run-time exception ArrayIndexOutOfBoundsException.

public synchronized void removeAll () (New)

The removeAll() method removes every option from the Choice. This allows you to refresh the list from scratch, rather than creating a new Choice and repopulating it.

Selection

The Choice has one item selected at a time. Initially, it is the first item that was added to the Choice.

public String getSelectedItem ()

The getSelectedItem() method returns the currently selected item as a String. The text returned is the parameter used in the addItem() or add() call that put the option in the Choice. If Choice is empty, getSelectedItem() returns null.

public Object[] getSelectedObjects () (New)

The getSelectedObjects() method returns the currently selected item as an Object array, instead of a String. The array will either be a one-element array, or null if there are no items. This method is required by the ItemSelectable interface and allows you to use the same method to look at the items selected by a Choice, List, or Checkbox.

public int getSelectedIndex ()

The getSelectedIndex() method returns the position of the currently selected item. The Choice list uses zero-based indexing, so the position of the first item is zero. The position of the last item is the value of countItems()-1. If the list is empty, this method returns -1.

public synchronized void select (int position)

This version of the select() method makes the item at position the selected item in the Choice. If position is too big, select() throws the run-time exception IllegalArgumentException. If position is negative, nothing happens.

public void select (String string)

This version of select() makes the item with the label string the selected item. If string is in the Choice multiple times, this method selects the first. If string is not in the Choice, nothing happens.

Miscellaneous methods

public synchronized void addNotify ()

The addNotify() method creates the Choice's peer. If you override this method, call super.addNotify() first, then add your customizations for the new class. You will then be able to do everything you need with the information about the newly created peer.

protected String paramString ()

When you call the toString() method of a Choice, the default toString() method of Component gets called. This in turn calls paramString() which builds up the string to display. At the Choice level, paramString() appends the currently selected item (the result of getSelectedItem()) to the output. Using the first Choice instance in Figure 9.1, the results would be:

java.awt.Choice[139,5,92x27,current=Dialog]

Choice Events

The primary event for a Choice occurs when the user selects an item in the list. With the 1.0 event model, selecting an item generates an ACTION_EVENT, which triggers a call to the action() method. Once the Choice has the input focus, the user can change the selection by using the arrow or keyboard keys. The arrow keys scroll through the list of choices, triggering the KEY_ACTION, ACTION_EVENT, and KEY_ACTION_RELEASE event sequence, which in turn invokes the keyDown(), action(), and keyUp() methods, respectively. If the mouse is used to choose an item, no mouse events are triggered as you scroll over each item, and an ACTION_EVENT occurs only when a specific choice is selected.

With the 1.1 event model, you register ItemListener with addItemListener(). Then when the user selects the Choice, the ItemListener.itemStateChanged() method is called through the protected Choice.processItemEvent() method. Key, mouse, and focus listeners are registered through the Component methods of addKeyListener(), addMouseListener(), and addFocusListener(), respectively. Action

public boolean action (Event e, Object o)

The action() method for a choice signifies that the user selected an item. e is the Event instance for the specific event, while o is the String from the call to addItem() or add() that represents the current selection. Here's a trivial implementation of the method:

public boolean action (Event e, Object o) {
    if (e.target instanceof Choice) {
        System.out.println ("Choice is now set to " + o);
    }
    return false;
}
Keyboard

The keyboard events for a Choice can be generated once the Choice has the input focus. In addition to the KEY_ACTION and KEY_ACTION_RELEASE events you get with the arrow keys, an ACTION_EVENT is generated over each entry.

public boolean keyDown (Event e, int key)

The keyDown() method is called whenever the user presses a key and the Choice has the input focus. e is the Event instance for the specific event, while key is the integer representation of the character pressed. The identifier for the event (e.id) for keyDown() could be either Event.KEY_PRESS for a regular key or Event.KEY_ACTION for an action-oriented key (i.e., arrow or function key). If you check the current selection in this method through the method getSelectedItem() or getSelectedIndex(), you will be given the previously selected item because the Choice's selection has not changed yet. keyDown() is not called when the Choice is changed by using the mouse.

public boolean keyUp (Event e, int key)

The keyUp() method is called whenever the user releases a key. e is the Event instance for the specific event, while key is the integer representation of the character pressed. The identifier for the event (e.id) for keyUp() could be either KEY_RELEASE for a regular key or KEY_ACTION_RELEASE for an action oriented key (i.e., arrow or function key).

Mouse

Ordinarily, the Choice component does not trigger any mouse events. Focus

Ordinarily, the Choice component does not trigger any focus events. Listeners and 1.1 event handling

With the 1.1 event model, you register listeners for different event types; the listeners are told when the event happens. These methods register listeners, and let the Choice component inspect its own events.

public void addItemListener(ItemListener listener) (New)

The addItemListener() method registers listener as an object interested in being notified when an ItemEvent passes through the EventQueue with this Choice as its target. The listener.itemStateChanged() method is called when an event occurs. Multiple listeners can be registered.

public void removeItemListener(ItemListener listener) (New)

The removeItemListener() method removes listener as a interested listener. If listener is not registered, nothing happens.

protected void processEvent(AWTEvent e) (New)

The processEvent() method receives all AWTEvents with this Choice as its target. processEvent() then passes them along to any listeners for processing. When you subclass Choice, overriding processEvent() allows you to process all events yourself, before sending them to any listeners. In a way, overriding processEvent() is like overriding handleEvent() using the 1.0 event model.

If you override processEvent(), remember to call super.processEvent(e) last to ensure that regular event processing can occur. If you want to process your own events, it's a good idea to call enableEvents() (inherited from Component) to ensure that events are delivered even in the absence of registered listeners.

protected void processItemEvent(ItemEvent e) (New)

The processItemEvent() method receives all ItemEvents with this Choice as its target. processItemEvent() then passes them along to any listeners for processing. When you subclass Choice, overriding processItemEvent() allows you to process all events yourself, before sending them to any listeners. In a way, overriding processItemEvent() is like overriding handleEvent() using the 1.0 event model.

If you override processItemEvent(), remember to call the method super.processItemEvent(e) last to ensure that regular event processing can occur. If you want to process your own events, it's a good idea to call enableEvents() (inherited from Component) to ensure that events are delivered even in the absence of registered listeners.

The following simple applet below demonstrates how a component can receive its own events by overriding processItemEvent(), while still allowing other objects to register as listeners. MyChoice11 is a subclass of Choice that processes its own item events. choice11 is an applet that uses the MyChoice11 component and registers itself as a listener for item events.

// Java 1.1 only
import java.awt.*;
import java.applet.*;
import java.awt.event.*;
class MyChoice11 extends Choice {
    MyChoice11 () {
        super ();
        enableEvents (AWTEvent.ITEM_EVENT_MASK);
    }
    protected void processItemEvent(ItemEvent e) {
        ItemSelectable ie = e.getItemSelectable();
        System.out.println ("Item Selected: " + ie.getSelectedObjects()[0]);
        // If you do not call super.processItemEvent()
        // no listener will be notified
        super.processItemEvent (e);
    }
}
public class choice11 extends Applet implements ItemListener {
    Choice c;
    public void init () {
        String []fonts;
        fonts = Toolkit.getDefaultToolkit().getFontList();
        c = new MyChoice11();
        for (int i = 0; i < fonts.length; i++) {
            c.add (fonts[i]);
        }
        add (c);
        c.addItemListener (this);
   }
    public void itemStateChanged(ItemEvent e)  {
        ItemSelectable ie = e.getItemSelectable();
        System.out.println ("State Change: " + ie.getSelectedObjects()[0]);
    }
}

A few things are worth noticing. MyChoice11 calls enableEvents() in its constructor to make sure that item events are delivered, even if nobody registers as a listener: MyChoice11 needs to make sure that it receives events, even in the absence of listeners. Its processItemEvent() method ends by calling the superclass's processItemEvent() method, with the original item event. This call ensures that normal item event processing occurs; super.processItemEvent() is responsible for distributing the event to any registered listeners. The alternative would be to implement the whole registration and event distribution mechanism inside myChoice11, which is precisely what object-oriented programming is supposed to avoid, or being absolutely sure that you will only use MyChoice11 in situations in which there won't be any listeners, drastically limiting the usefulness of this class.

choice11 doesn't contain many surprises. It implements ItemListener, the listener interface for item events; provides the required itemStateChanged() method, which is called whenever an item event occurs; and calls MyChoice11's method addItemListener() to register as a listener for item events. (MyChoice11 inherits this method from the Choice class.)


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