The synchronized lock the reference so if you change the reference an other thread could be execute the synchronized block in same time

In French : https://www.jmdoudoux.fr/java/dej/chap-acces_concurrents.htm#:~:text=Il%20ne%20faut%20pas%20utiliser,pos%C3%A9%20sur%20le%20m%C3%AAme%20moniteur.