Listener not reacting to second call - kotlin

Question

I'm trying to learn Kotlin and I'm writing code to test and confirm my understanding. In the result of the following code, I expect to see "Property age changed from 37 to 38" near the end. Why am I not seeing that.
I tried writing the increment to age as age.plus initially, but that made the same result.
package ch07.ImplementingDelegatedProperties
import java.beans.PropertyChangeSupport
import java.beans.PropertyChangeListener
open class PropertyChangeAware {
protected val changeSupport = PropertyChangeSupport(this)
fun addPropertyChangeListener(listener: PropertyChangeListener) {
changeSupport.addPropertyChangeListener(listener)
}
fun removePropertyChangeListener(listener: PropertyChangeListener) {
changeSupport.removePropertyChangeListener(listener)
}
}
class Person(
val name: String, age: Int?, salary: Int
)
: PropertyChangeAware()
{
var age: Int? = age
set(newValue) {
println("age changed")
val oldValue = field
field = newValue
changeSupport.firePropertyChange(
"age", oldValue, newValue)
}
var salary: Int = salary
set(newValue) {
val oldValue = field
field = newValue
changeSupport.firePropertyChange(
"salary", oldValue, newValue)
}
operator fun plus (older: Int) :Person {
println("age changed by operator")
val oldValue = age
val newValue = (oldValue?: 0) + older
changeSupport.firePropertyChange( // don't know why this dosn't fire repeatedly
"age", oldValue, newValue)
println("age chg by operator, $oldValue -> $newValue")
this.age = (oldValue?: 0) + older
// this._attributes = _attributes
return Person(name, age = age, salary = salary)
}
}
fun main(args: Array<String>) {
var p = Person("Dmitry", 34, 2000)
p.addPropertyChangeListener(
PropertyChangeListener { event ->
println("Property ${event.propertyName} changed " +
"from ${event.oldValue} to ${event.newValue}")
}
)
println(p.age)
p.age = 35
println(p.age.toString())
p.age = 36
println(p.age.toString())
p = p + 1
println(p.age.toString())
p = p + 1
println(p.age.toString())
p.salary = 2100
}
Actual results:
34
age changed
Property age changed from 34 to 35
35
age changed
Property age changed from 35 to 36
36
age changed by operator
Property age changed from 36 to 37
age chg by operator, 36 -> 37
age changed
Property age changed from 36 to 37
37
age changed by operator
age chg by operator, 37 -> 38
age changed
38
Expected results:
34
age changed
Property age changed from 34 to 35
35
age changed
Property age changed from 35 to 36
36
age changed by operator
Property age changed from 36 to 37
age chg by operator, 36 -> 37
age changed
Property age changed from 36 to 37
37
age changed by operator
age chg by operator, 37 -> 38
age changed
Property age changed from 37 to 38 << This line expected but not appearing
38

Answer 1

The problem is that, at the end of your plus function, you create a brand new Person object (which copies the name and salary and increases the age) but importantly, it has a new changeSupport object which you haven't run addPropertyChangeListener on.
It looks like PropertyChangeSupport allows you to have no listeners defined, so won't error or give you a clue that there is not listener