The following methods are synchronized:
 addPropertyChangeListener(PropertyChangeListener)
 removePropertyChangeListener(PropertyChangeListener)
 getPropertyChangeListeners()
 addPropertyChangeListener(String,PropertyChangeListener)
 void removePropertyChangeListener(String,PropertyChangeListener)
 getPropertyChangeListeners(String)
 hasListeners(String)
Method firePropertyChange(PropertyChangeEvent) is not synchronized,
but it accesses to guarded fields: listeners and children.