I got
$ sudo pacman -Syu jdk-openjdk
:: Synchronizing package databases...
mesa-nonfree is up to date
core is up to date
extra is up to date
community is up to date
multilib is up to date
repo-ck is up to date
:: Starting full system upgrade...
resolving dependencies...
looking for conflicting packages...
error: unresolvable package conflicts detected
error: failed to prepare transaction (conflicting dependencies)
:: jdk-openjdk and jre-openjdk are in conflict