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To get the most out of this section, first read Chapters 7, Properties, and Chapter 8, Introspection, of theJavaBeans API Specification
.
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:
From this the builder tool infers that a property named color exists, that it is readable and writeable, and that it's type ispublic Color getColor() { ... } public void setColor(Color c) { ... }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.
Make the following changes to
SimpleBean.java
to add a color property:
- Create and initialize a private instance variable.
private Color color = Color.green;- Write a
public
getter method.public Color getColor(){ return color; }- Write a
public
setter method.public void setColor(Color newColor){ color = newColor; repaint(); }- Override the
paint()
method inherited fromCanvas
.public void paint(Graphics g) { g.setColor(color); g.fillRect(20, 5, 20, 30); }- 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 aColor
property editor. AColor
property editor is one of the default editors supplied with the BeanBox, and is assigned asSimpleBean
'sColor
property editor. Click on theColor
property in the Properties sheet to run this editor. Section 9.2 of theJavaBeans API Specification
discusses property editors.
The following figure shows the revised
SimpleBean
instance within the BeanBox,SimpleBean
's newColor
property within the Properties sheet, and theColor
property editor shipped with the BeanBox.
This figure has been reduced to fit on the page.
Click the image to view it at its natural size.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 acolor
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); } }
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