Writing Advanced Beans

Implementing Indexed Properties

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To get the most out of this section, read the following documentation:

• Section 7.2 of the JavaBeans API Specification.
• IndexedPropertyDescriptor class
• PropertyEditor interface
• PropertyEditorSupport class

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Indexed properties represent collections of values accessed, like an array, by index. The indexed property design patterns are

//Methods to access the entire indexed property array
public <PropertyType>[] get();
public void set<PropertyName>([] value);

//Methods to access individual values
public <PropertyType> get(int index);
public void set<PropertyName>(int index,  value);

Conforming to these patterns lets builder tools know that your Bean contains an indexed property.

The OurListBox demo Bean illustrates how to use an indexed property. OurListBox extends the List class to provide a Bean that presents the user a list of choices: Choices that you can provide and change at design time. Here's an illustration of an OurListBox instance:

OurListBox exposes the item indexed property with the following accessor methods:

public void setItems(String[] indexprop) {
  String[] oldValue=fieldIndexprop;
  fieldIndexprop=indexprop;
  populateListBox();
  support.firePropertyChange("items",oldValue, indexprop);
}

public void setItems(int index, String indexprop) {
  String[] oldValue=fieldIndexprop;
  fieldIndexprop[index]=indexprop;
  populateListBox();
  support.firePropertyChange("Items",oldValue, fieldIndexprop);
}
 
public String[] getItems() {
  return fieldIndexprop;
}
    
public String getItems(int index) {
  return getItems()[index];
}

When an item is set by one of the setItems() methods, OurListBox is populated with the contents of a String array.

Indexed properties are almost as easily exposed as simple properties. Writing an indexed property editor, though, requires writing a custom property editor.

Indexed Property Editors

The OurListBox demo Bean provides an associated IndexPropertyEditor which is a good example of how to implement an indexed property editor. The following illustration shows an OurListBox instance in the BeanBox, the Properties sheet which contains an entry for the indexed property items, and the IndexPropertyEditor which pops up when the items property entry is clicked:

Implementing IndexPropertyEditor is the same as implementing any custom property editor:

1. Implement the PropertyEditor interface:

          public class IndexPropertyEditor extends Panel
                        implements PropertyEditor, Action Listener {
         

   You can use the PropertyEditorSupport class, either by subclassing or as an inner class.
2. Denote the custom editor in a related BeanInfo class. OurListBox has a related OurListBoxBeanInfo class that contains the following code:

            itemsprop.setPropertyEditorClass(IndexPropertyEditor.class);
         

3. Make the property editor a source for bound property events. The property editor will register property listeners, and fire property change events at those listeners. This is how the property changes are propagated back to the Bean (via the property sheet). So IndexPropertyEditor instantiates an inner PropertyChangeSupport class:

             private PropertyChangeSupport support =
                  new PropertyChangeSupport(this);
          

   Provides the ability for objects to register their interest in being notified when a property is edited:

          public void addPropertyChangeListener(PropertyChangeListener l) {
           support.addPropertyChangeListener(l);
          }
   
          public void removePropertyChangeListener(PropertyChangeListener l) {
           support.removePropertyChangeListener(l);
          }
          

   And fires property change events at those listeners:

             public void actionPerformed(ActionEvent evt) {
           if (evt.getSource() == addButton) {
               listBox.addItem(textBox.getText());
               textBox.setText("");
               support.firePropertyChange("", null, null);
           }
           else if (evt.getSource()== textBox) {
               listBox.addItem(textBox.getText());
               textBox.setText("");
               support.firePropertyChange("",null,null);
           }
           ...
          }
          

IndexPropertyEditor maintains listbox as a proxy for OurListBox. When a change is made to listbox, a property change event is fired to all listeners.

When the Properties sheet, which is registered as an IndexPropertyEditor listener, receives a property change event from IndexPropertyEditor, the Properties sheet calls IndexPropertyEditor.getValue() to retrieve the new or changed items and update the Bean.

Writing Advanced Beans