|Lesson 5||Property design patterns|
|Objective|| How do you define and use "Property Design Patterns"?|
How do you define and use "Property Design Patterns"?
Property design patterns are used to identify the properties of a Bean. Not surprisingly, property design patterns are closely related to accessor methods.
In fact, accessor methods are the means by which the JavaBean's automatic introspection facility determines the properties of a Bean.
Basically, any time the JavaBeans introspector encounters a public
getter or setter method, it assumes the member variable being get or set is a property, and then exposes the property to the outside world.
Types of property design patterns
The design patterns for properties vary a little based on the type of property. Following are the different property design patterns:
- Simple property design patterns
- Boolean property design patterns
- Indexed property design patterns
At first glance it appears that you will have to implement the entire java.beans.BeanInfo interface even if you have a very simple Bean that only requires a few of the different descriptors. Actually, each of the methods in the interface are free to return null, indicating that the caller should use low-level reflection to gather the information.
To make it easier to implement simple BeanInfo classes, the class
java.beans.SimpleBeanInfo implements all of the interfaces of the java.beans.BeanInfo interface. Each method denies any knowledge of the associated data, suggesting that reflection should be used. You can derive your own BeanInfo classes by extending java.beans.SimpleBeanInfo, overriding those methods that return the information that you want to explicitly specify.
Another important aspect of providing explicit introspection information is information about base classes. If low-level reflection is used to analyze a Bean, all of its base classes (superclasses) are also analyzed by walking upwards through the class hierarchy. Each base class is analyzed in turn. The information gathered while analyzing a particular class is considered to be definitive. This means that if property information is found in a base class and that has already been discovered in a subclass, the subclass information takes precedence. For example, let's say class A has a read-only property called Value, and class B extends class A and implements the public methods getValue() and setValue(). The information in class B takes precedence over the information in class A, so any Bean based on class B will have a read/write property called Value, while the Value property of a Bean based on class A will be read-only.
Property Design - Exercise
Click the Exercise link to define accessor methods based on property design patterns.
Property Design - Exercise
In the next lesson, multicast event design patterns
will be discussed.
Developing Java Beans
JavaBeans Quiz Question
Which of the given options contain the correct set of methods for getting and setting the value of a boolean property 'bright', according to JavaBeans naming standards?
Select two choices.
a. public void setBright(boolean bright)
public boolean getBright()
b. public void setBright(boolean bright)
public boolean isBright()
c. public void setbright(boolean bright)
public boolean getbright()
d. public void setbright(boolean bright)
public boolean isbright()
Answer : a, b
Choices A and B are the correct answers. According to the JavaBeans naming standards, if the property name is 'x' and the type is Type, the accessor method is of the form:
and the mutator method is of the form:
void setX(Type newValue)
However, boolean property also uses another convention:
void setX(boolean newValue)
So choice C is incorrect, while choices A and B are correct. The name of the property is capitalized in the accessor and mutator methods.
So choice D is incorrect.