Properties

Simple Properties

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To get the most out of this section, first read Chapters 7, Properties, and Chapter 8, Introspection, of the JavaBeans API Specification.

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Properties are aspects of a Bean's appearance and behavior that are changeable at design time. Properties are private values accessed through getter and setter methods. Property getter and setter method names follow specific rules, called design patterns. By using these design pattern-based method names, JavaBeans-enabled builder tools (and the BeanBox) can

• Discover a Bean's properties
• Determine the properties' read/write attributes
• Determine the properties' types
• Locate an appropriate property editor for each property type
• Display the properties' (usually in a property sheet)
• Alter those properties (at design time)

For example, a builder tool, on introspecting your Bean, discovers two methods:

public Color getColor() { ... }
public void setColor(Color c) { ... }

From this the builder tool infers that a property named color exists, that it is readable and writeable, and that it's type is Color. Further, the builder tool can attempt to locate a property editor for that type, and display the property (usually in a property sheet) so it can be edited.

Adding a Color Property to SimpleBean

Make the following changes to SimpleBean.java to add a color property:

1. Create and initialize a private instance variable.

      private Color color = Color.green; 
     

2. Write a public getter method.

      public Color getColor(){
        return color;
      } 
     

3. Write a public setter method.

       public void setColor(Color newColor){
        color = newColor;
        repaint();
       }
      

4. Override the paint() method inherited from Canvas.

       public void paint(Graphics g) {
        g.setColor(color);
        g.fillRect(20, 5, 20, 30);
       }
      

5. Compile the Bean, load it in the ToolBox, and create an instance in the BeanBox.

The results are:

• SimpleBean will be displayed with a green centered rectangle.
• The Properties sheet will contain a new Color property. The introspection mechanism will also search for a Color property editor. A Color property editor is one of the default editors supplied with the BeanBox, and is assigned as SimpleBean's Color property editor. Click on the Color property in the Properties sheet to run this editor. Section 9.2 of the JavaBeans API Specification discusses property editors.

The following figure shows the revised SimpleBean instance within the BeanBox, SimpleBean's new Color property within the Properties sheet, and the Color property editor shipped with the BeanBox.

You can change the color property by menu, or by RGB value. Try changing colors.

Here is the complete SimpleBean source code, revised to add a color property.

package sunw.demo.simple;
 
import java.awt.*;
import java.io.Serializable;
 
public class SimpleBean extends Canvas
                     implements Serializable{
 
  private Color color = Color.green;
 
  //property getter method
  public Color getColor(){
     return color;
  }
 
  //property setter method. Sets new SimpleBean
  //color and repaints.
  public void setColor(Color newColor){
     color = newColor;
     repaint();
  }
 
  public void paint(Graphics g) {
   g.setColor(color);
   g.fillRect(20, 5, 20, 30);
  }
 
  //Constructor sets inherited properties
  public SimpleBean(){
   setSize(60,40);
   setBackground(Color.red);
  }
}

Properties