ADDITIONAL SYSTEM INFORMATION :
reproduced on opensuse leap 42.3 and oS tumbleweed 20180410

A DESCRIPTION OF THE PROBLEM :
    alt_install_java() {}
and alt_install_javac attempts to mark javaws as a slave in /etc/alterantives but it is already a slave of javaplugins causing the installation to fail on opensuse leap 42.3 and tumbleweed.
the same issue was mentioned in 216 https://en.opensuse.org/SDB_Talk:Installing_Java

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1)system with openjdk and alternatives already installed.
2)download the oracle JDK 64 bit binary package
execute zypper install or rpm -ivh on the package
the post installation scrip will fail on these two errors, java and java compilers are not set.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
the alternatives would be set