Java beans

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

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’.

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;
}
}

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

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

Read-only properties – only getter method
Write-only properties – only setter method
Read/write properties – getter & setter

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

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

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; }
}

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”
}
}

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

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

Bound properties -
Student.java
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

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

Bound properties –
Notify listeners
Student.java
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);

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

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);
}
}

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

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

Constrained properties -
Student.java
private int total;
//getters & setters
VetoableChangeSupport vcs = new
VetoableChangeSupport(this);
public void
addVetoableChangeListener(VetoableChangeListener pcl) {
this.vcs.addVetoableChangeListener(pcl);
}
//remove vetoable change listener

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

Constrained properties –
Notify listeners
Student.java
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);

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

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);
}
}
}
}

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

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

– XMLEncoder
Student.java
private String[] subjectsTaken;
//getters & setters
}

– 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();

– 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>

– XMLDecoder
StudentDecoder.java
XMLDecoder decoder = new XMLDecoder(
new BufferedInputStream(
new FileInputStream(“Student.xml”)));
Student student = (Student) decoder.getObject();

Java beans

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