Java Beans allow developers to capture and discover the state of objects by defining classes that follow the Java Bean specification. A Java class is considered a Java Bean if it contains getter and/or setter methods, does not extend other classes or interfaces, and follows other naming conventions. Properties of a Java Bean are any aspects that can be accessed via getter and setter methods. There are different types of properties such as indexed, bound, and constrained properties. Java Beans also support long term persistence by encoding and decoding bean objects from XML files.

1. Java Beans
Shivasubramanian A

2. What is a Java Bean?
 Ensures that developers can
 Capture the state of a given entity
 Discover the state of a given entity
 Achieved by writing Java classes that follow
the JavaBean specification
 Belongs to the java.desktop module
 Two packages
 java.beans
 java.beans.context

3. When is a Java class a JavaBean?
 When the class follows the JavaBean spec
 The class should only contain getters and/or
setters.
 The class should not extend any other class or
interface.
 Getters returning boolean values must begin with
‘is’.

4. When is a Java class a JavaBean?
class Employee {
private Integer age;
public void getAge() { return this.age; }
public void setAge(Integer age) {
this.age = age;
}
}

5. Java Bean properties
 Any aspect of a Java Bean available via
getter/setter methods is called a “Java bean
property”.

6. Java Bean properties
class Employee {
private Integer age;
public void getAge() { return this.age; }
public void setAge(Integer age) {
this.age = age;
}
public void isValidAge() {
return age <= 100 && age >= 0;
}
}
Properties – age, validAge

7. Java Bean properties
Read-only properties – only getter method
Write-only properties – only setter method
Read/write properties – getter & setter

8. Types of Java Bean properties
Indexed properties
Bound properties
Constrained properties

9. Indexed properties
Basically arrays
 Getters return array
 Setters take in an array
Must provide getter/setter for each element of
the property

10. Indexed properties...
Student.java
public class Student {
private String name;
private String[] subjects = new String[] { “Maths”,
“Science”, “Geography”};
public void getSubjects { return this.subjects;}
public void setSubjects(String[] sub) { this.subjects =
sub; }
public void getSubject(int index) { return
this.subjects[index]; }
public void setSubject(int index, String subject) {
this.subjects[index] = subject; }
}

11. Indexed properties...
Main.java
public class Main {
public static void main(String args[]) {
Student student = new Student();
student.getSubject(2); //returns “Geography”
student.setSubject(2, “Civics”);
student.getSubject(2); //returns “Civics”
}
}

12. Bound properties
Java bean which has listeners
 Changes to the bean triggers the listeners
You have to write code to:
 Add/remove listeners
 Notify listeners
You can have listeners for
 All properties
 Specific properties

13. Bound properties -
Add/remove listeners
java.beans.PropertyChangeSupport
 Provides methods to maintain a list of
listeners
 Provides methods to notify listeners of
changes

14. Bound properties -
Add/remove listeners
Student.java
public class Student {
private String name;
private int[] marks = new int[3];
private int total;
//getters & setters
PropertyChangeSupport pcs = new
PropertyChangeSupport(this);
public void addPropertyListener(PropertyChangeListener pcl)
{
this.pcs.addPropertyChangeListener(pcl);
}
//remove property listener

15. Bound properties –
Notify listeners
To notify listeners, use methods
 PropertyChangeSupport.firePropert
yChange
 PropertyChangeSupport.fireIndexed
PropertyChange

16. Bound properties –
Notify listeners
Student.java
public class Student {
private String name;
private int[] marks = new int[3];
private int total;
//add/remove property listener
PropertyChangeSupport pcs = new
PropertyChangeSupport(this);
public void setMarks(int[] marks) {
this.marks = marks;
int oldTotal = total;
this.total = sum(marks);
pcs.firePropertyChange(“total”, oldTotal, this.total);

17. Bound properties – listener code
Listener classes
 Implements PropertyChangeListener
 Single method,
propertyChange(PropertyChangeEven
t evt)

18. Bound properties – listener code
StudentRanker.java
public class StudentRanker implements PropertyChangeListener {
private List<Student> studentsRanking = new ArrayList<>();
public void propertyChange(PropertyChangeEvent evt) {
Student source = (Student) evt.getSource();
studentsRanking.add(source);
studentsRanking.sort(studentsTotalComparator);
}
}

19. Constrained properties
Bound properties that have veto power over
the new value being set
 Throw
java.beans.PropertyVetoException
to veto
You have to write code to:
 Add/remove listeners
 Notify listeners
You can have listeners for

20. Constrained properties -
Add/remove listeners
java.beans.VetoableChangeSupport
 Provides methods to maintain a list of
listeners
 Provides methods to notify listeners of
changes

21. Constrained properties -
Add/remove listeners
Student.java
public class Student {
private String name;
private int[] marks = new int[3];
private int total;
//getters & setters
VetoableChangeSupport vcs = new
VetoableChangeSupport(this);
public void
addVetoableChangeListener(VetoableChangeListener pcl) {
this.vcs.addVetoableChangeListener(pcl);
}
//remove vetoable change listener

22. Constrained properties –
Notify listeners
To notify listeners, use methods
 PropertyChangeSupport.fireVetoabl
eChange

23. Constrained properties –
Notify listeners
Student.java
public class Student {
private String name;
private int[] marks = new int[3];
private int total;
//add/remove property listener
VetoableChangeSupport vcs = new
VetoableChangeSupport(this);
public void setMarks(int[] marks) {
vcs.fireVetoableChange(“marks”, this.marks, marks);
this.marks = marks;
int oldTotal = total;
this.total = sum(marks);

24. Constrained properties – listener
code
Listener classes
 Implements VetoableChangeListener
 Single method,
vetoableChange(PropertyChangeEven
t evt)

25. Constrained properties – listener
code
MarksValidator.java
public class MarksValidator implements VetoableChangeListener {
public void propertyChange(PropertyChangeEvent evt) {
int[] marks = (int[]) evt.getNewValue();
for (int mark : marks) {
if (mark < 0 || mark > 100) {
throw new PropertyVetoException(“Mark is
invalid.”, evt);
}
}
}
}

26. Bean methods
Any methods that don’t meet the JavaBean
spec are called bean methods

27. Long term bean persistence
Store a bean as an XML file
Useful for sharing state of beans
java.beans.XMLEncoder will convert the bean
into an XML file
java.beans.XMLDecoder will build the bean
from the XML file

28. Long term bean persistence
– XMLEncoder
Student.java
public class Student {
private String name;
private String[] subjectsTaken;
//getters & setters
}

29. Long term bean persistence
– XMLEncoder
StudentEncoder.java
Student student = new Student();
student.setName(“Student1”);
student.setSubjects(new String[] {“Maths”, “Physics”});
XMLEncoder encoder = new XMLEncoder(
new BufferedOutputStream(
new FileOutputStream(“Student.xml”)));
encoder.writeObject(student);
encoder.close();

30. Long term bean persistence
– XMLEncoder
Student.xml
<object class=”Student”>
<void property=”name”>
<string>Student1</string>
</void>
<void property=”subjectsTaken”>
<array class=”java.lang.String” length=”2”>
<void index=”0”>
<string>Maths</string>
</void>
<void index=”1”>
<string>Science</string>

31. Long term bean persistence
– XMLDecoder
StudentDecoder.java
XMLDecoder decoder = new XMLDecoder(
new BufferedInputStream(
new FileInputStream(“Student.xml”)));
Student student = (Student) decoder.getObject();