Java8 and Apple Java SE 6 are installed in two different places. You will have to either remove Java SE6, or "link" (Unix links) to the Oracle Java8, once it is installed.
For example this machine has no Java installed (it only has the Apple Java SE6 skeleton, there is no JRE/JDK), and redirects to Oracle Java
type java
java is /usr/bin/java
ls -ld /usr/bin/java
lrwxr-xr-x 1 root wheel 74 Sep 11 15:40 /usr/bin/java -> /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java
ls /System/Library/Frameworks/
Display all 126 possibilities? (y or n)
ls /System/Library/Frameworks/JavaVM.framework/
Frameworks/ JavaVM Resources/ Versions/ module.map
ls /System/Library/Frameworks/JavaVM.framework/Versions/
A Current
ls -l /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/Java
-rwxr-xr-x 1 root wheel 50160 Aug 8 09:00 /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/Java