This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The error message with your Exception tells you what you need to know.
Whichever object you have as equityTicker (BTW: why are you using direct access to a field?) does not implement the Comparable<T> interface and therefore does not have a compareTo(T t) method. You might have a spelling error that you are implementing Comparator instead of Comparable<T>.
I cannot see an explicit cast operator in any of the code you have quoted, so you have probably not yet found the line where the exception took place.