[Chapter 10] 10.6 Defining a Simple Property Editor

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| Chapter 10 Java Beans |

## 10.6 Defining a Simple Property Editor

A bean can also provide an auxiliary `PropertyEditor` for use by a beanbox tool. `PropertyEditor` is a flexible interface that allows a bean to tell a beanbox how to display and edit the values of certain types of properties.

A beanbox tool always provides simple property editors for common property types, such as strings, numbers, fonts, and colors. If your bean has properties of a non-standard type, you should register a property editor for that type. The easiest way to "register" a property editor is through a simple naming convention. If your type is defined by the class `X`, the editor for it should be defined in the class `X` `Editor`. Alternately, you can register a property editor by calling the `PropertyEditorManager.registerEditor()` method, probably from the constructor of your `BeanInfo` class. If you call this method from the bean itself, the bean then depends on the property editor class, so the editor has to be bundled with the bean in applications.

In our `YesNoDialog` example, we don't define any new data types, but we still have two individual properties that need custom editors. In this case, we register the property editors for individual properties by specifying them in the `PropertyDescriptor` objects returned by the `getPropertyDescriptors()` method of our `BeanInfo` class.

The `PropertyEditor` interface can seem confusing at first. Its methods allow you to define three techniques for displaying the value of a property and two techniques for allowing the user to edit the value of a property. The value of a property can be displayed:

- As a string. If you define the `getAsText()` method, a beanbox can convert a property to a string and display that string to the user.

- As an enumerated value. If a property can only take on a value from a fixed set of values, you can define the `getTags()` method to allow a beanbox to display a dropdown menu of allowed values for the property.

- In a graphical display. If you define `paintValue()`, a beanbox can ask the property editor to display the value using some natural graphical format, such as a color swatch for colors. You also need to define `isPaintable()` to specify that a graphical format is supported.

The two editing techniques are:

- String editing. If you define the `setAsText()` method, a beanbox knows it can simply have the user type a value into a text field and pass that value to `setAsText()`. If your property editor defines `getTags()`, it should also define `setAsText()` so that a beanbox can set the property value using the individual tag values.

- Custom editing. If your property editor defines `getCustomEditor()`, a beanbox can call it to obtain some kind of AWT component that can be displayed in a dialog box and serve as a custom editor for the property. You also need to define `supportsCustomEditor()` to specify that custom editing is supported.

The `setValue()` method of a `PropertyEditor` is called to specify the current value of the property. It is this value that should be converted to a string or graphical representation by `getAsText()` or `paintValue()`.

A property editor is required to maintain a list of event listeners that are interested in changes to the value of the property. The `addPropertyChangeListener()` and `removePropertyChangeListener()` methods are standard event listener registration and removal methods. When a property editor changes the value of a property, either through `setAsText()` or through a custom editor, it must send a `PropertyChangeEvent` to all registered listeners.

`PropertyEditor` defines the `getJavaInitializationString()` for use by beanbox tools that generate Java code. This method should return a fragment of Java code that can be used to initialize a variable to the current property value.

Finally, a class that implements the `PropertyEditor` interface must have a no-argument constructor, so that it can be dynamically loaded and instantiated by a beanbox.

Most property editors can be much simpler than this detailed description would suggest. In many cases, you can subclass `PropertyEditorSupport` instead of implementing the `PropertyEditor` interface directly. This useful class provides no-op implementations of most `PropertyEditor` methods. It also implements the methods for adding and removing event listeners.

A property that has an enumerated value requires a simple property editor. The `alignment` property of the `YesNoDialog` bean is an example of this common type of property. The property is defined as an `int`, but it has only three legal values, defined by the constants `LEFT`, `CENTER`, and `RIGHT`. By default, a beanbox only knows that the property is an `int`, so it displays the property as a number and allows the user to enter any integer as a property value. Instead, we would like the beanbox to display one of the strings "left," "center," or "right" as the value, and allow the user to choose from these values when setting the property. This can be done with the `getTags()` and `setAsText()` methods of a property editor, as shown in Example 10.6.

This example creates the `YesNoDialogAlignmentEditor` class, which is registered as a `PropertyEditor` for the `alignment` property by the `YesNoDialogBeanInfo` class shown in Example 10.5 . The property editor subclasses `PropertyEditorSupport`, so it is relatively short. Notice that it passes `Integer` objects in the calls to `setValue()` that are made from the `setAsText()` method. You need to use wrapper objects for any primitive-type properties. The use of the `Integer` class is also apparent in the definition of `getJavaInitializationString()`. The `setValue()` method of `ProperyEditorSupport` handles notifying registered `PropertyChangeListener` objects about changes in the value of the property, so this simple property editor does not need to be aware of the existence of such listeners.

#### Example 10.6: The YesNoDialogAlignmentEditor Class

    package oreilly.beans.yesno;
    import java.beans.*;
    import java.awt.*;
    public class YesNoDialogAlignmentEditor extends PropertyEditorSupport {
      // These two methods allow the property to be edited in a dropdown list.
      // Return the list of value names for the enumerated type.
      public String[] getTags() {
        return new String[] { "left", "center", "right" };
      }
      // Convert each of those value names into the actual value.
      public void setAsText(String s) {
        if (s.equals("left")) setValue(new Integer(YesNoDialog.LEFT));
        else if (s.equals("center")) setValue(new Integer(YesNoDialog.CENTER));
        else if (s.equals("right")) setValue(new Integer(YesNoDialog.RIGHT));
        else throw new IllegalArgumentException(s);
      }
      // This is an important method for code generation.
      public String getJavaInitializationString() {
        switch(((Number)getValue()).intValue()) {
        default:
        case YesNoDialog.LEFT:   return "oreilly.beans.yesno.YesNoDialog.LEFT";
        case YesNoDialog.CENTER: return "oreilly.beans.yesno.YesNoDialog.CENTER";
        case YesNoDialog.RIGHT:  return "oreilly.beans.yesno.YesNoDialog.RIGHT";
        }
      }
    }

Specifying Bean Information |  | Defining a Complex Property Editor