The javadoc command is run with the following option:

-link "http://download.oracle.com/javase/7/docs/api"

This should be:

-link "http://docs.oracle.com/javase/8/docs/api/"

Note that we get the following warning message from javadoc using the current (JDK 7) link.

javadoc: warning - Error fetching URL: http://download.oracle.com/javase/7/docs/api

It might be a problem with pointing to a redirected URL. If so, then using the actual JDK 8 link shown above might fix the problem (else we will need to diagnose it).