Swing Hacks

Create a Collections-Aware JComboBox #20

Chapter 2, Lists and Combos

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HACK

Using toString( ) on the strings will be the same as the default renderer,

simply drawing Proton, Neutron, and Electron in the list (see Figure 2-12).

The resulting change would look like Figure 2-13.

Using reflection for cell renderers is a very powerful concept because you

can reuse potentially complicated code with very little additional effort.

Using it to display strings is just a trivial example. Imagine you had a bunch

of objects representing entries from an RSS feed. Instead of creating a cus-

tom renderer or wrapping the entries in objects with a custom

toString( )

method, you could use the GenericListCellRenderer with getTitle( ) to

automatically call the

getTitle( ) method on the entry objects—no new

subclasses or extra code. Just a single string and the renderer takes care of

the rest. That is the power of reflection.

H A C K

#20

Create a Collections-Aware JComboBox Hack #20

You’ve moved on from Vector; your combo boxes should, too.

JComboBox is one of Swing’s oldest components. Unfortunately, it accepts

arrays of objects and

Vectors only. Now that Collections objects like List

have been part of the JDK for years, it would be nice to use them directly in

a combo box without shuffling objects in and out of arrays. Fortunately, the

JComboBox uses an MVC (Model-View-Controller) architecture, so you can

solve this problem with a simple implementation of a

ComboBoxModel.

Figure 2-12. Renderer using the toString( ) method

Figure 2-13. Renderer using the hashCode( ) method

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Chapter 2, Lists and Combos

#20 Create a Collections-Aware JComboBox

HACK

To start, you need to figure out what the custom model should do. For our

purposes, it needs to accept a

List in the constructor and preserve any

ordering supplied. Another nifty feature would be automatic updates. If you

add or delete values to the

List

, the combo box should update itself auto-

matically. Example 2-22 is a good start.

This implementation is pretty much what you’d expect. Each method in

ComboBoxModel is implemented (along with its parent interface,

ListDataModel). The constructor saves a reference to the List and selects the

first element if there is one. The

selectedItem accessor works as expected,

using the selected variable.

getElementAt( ) and getSize( ) both pass the

work on to the underlying

List, and the ListDataListener methods work

Example 2-22. A basic combo box to accept lists

public class ListComboBoxModel implements ComboBoxModel {

protected List data;

public ListComboBoxModel(List list) {

this.listeners = new ArrayList( );

this.data = list;

if(list.size( ) > 0) {

selected = list.get(0);

}

}

protected Object selected;

public void setSelectedItem(Object item) {

this.selected = item;

}

public Object getSelectedItem( ) {

return this.selected;

}

public Object getElementAt(int index) {

return data.get(index);

}

public int getSize( ) {

return data.size( );

}

protected List listeners;

public void addListDataListener(ListDataListener l) {

listeners.add(l);

}

public void removeListDataListener(ListDataListener l) {

this.listeners.remove(l);

}

}

Create a Collections-Aware JComboBox #20

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with a second List for managing the listeners. The important thing to notice

here is that the code saves the reference to the

List that was passed in,

rather than creating a copy. This means that the model will always be in

sync with the underlying list implementation. If you call

list.add("new

item")

, it will show up in the combo box automatically.

To test this, use the simple class in Example 2-23.

The program creates a

JComboBox that uses the new ListComboBoxModel. First,

it creates a list, populates it with data, passes it to the

ListComboBoxModel

constructor, and then sends that to the new JComboBox( ). There is also a

button that adds a new item to the list when clicked.

When you compile and run this program, it...doesn’t work! The addition to

the

List doesn’t show up in the combo box. A look over the API might

remind you of the

ListDataListener class. When setModel( ) is called, the

Example 2-23. Testing the List-based JComboBox

public class CBTest {

public static void main(String[] args) {

JFrame frame = new JFrame("Hack #4: Create a Collections-Aware

JComboBox");

Container root = frame.getContentPane( );

root.setLayout(new BoxLayout(root,BoxLayout.X_AXIS));

// List combo box

final List list = new ArrayList( );

list.add("Blinky");

list.add("Pinky");

list.add("Inky");

final ListComboBoxModel mod2 = new ListComboBoxModel(list);

JComboBox cb2 = new JComboBox( );

cb2.setModel(mod2);

root.add(cb2);

final JButton bt2 = new JButton("Add Item");

bt2.addActionListener(new ActionListener( ) {

public void actionPerformed(ActionEvent evt) {

list.add("Clyde");

}

});

root.add(bt2);

// show the frame

frame.pack( );

frame.setVisible(true);

}

}