Question

Why method firePropertyChange (String propertyName, Object oldValue, Object newValue) in class PropertyChangeSupport don’t check that old and new value can be null at the same time?

Answer 1

PropertyChangeEvent javadoc may give some clue about it:

Null values may be provided for the old and the new values if their
true values are not known.

An event source may send a null object as the name to indicate that an
arbitrary set of if its properties have changed. In this case the old
and new values should also be null.

So it looks like having oldValue==null and newValue==null can have some special meaning when source==null as well. Because of this is may want to always propagate the change when both values are nulls even though they are the same.