To preface my answer I will say I have worked in the finance industry so I will start with a little history lesson before going into why you should learn C++.
Before Java's release C++ was the language used primarily for banking applications, especially when dealing with low latency situations such as HFT. When Java became stable enough with delivering the "write one, run anywhere" slogan that the banks started to adopt it as the new default language for smaller application and, when it's performance improved, they began using it for larger ones.
With this in mind you should also know that banks are in habit of both writing all there own stuff by default (with some exceptions) and not re-writing it if it just works in making them money; you do see a lot of legacy applications that require ancient versions of the JVM or a certain amount of magic to get working but are adding business value. Older versions of the JVM had sucky performance compared to C++ at the time and indeed current generation VMs so writing a low latency, real time system was probably not seen as being particularly viable from a performance point of view.
In other words the odds are you will likely be maintaining an existing platform that was probably written when C++ was the language of choice. At the end of the day it will either be Java or C++, you might stumble across some C or even assembly (unlikely). Additionally even if the application is Java it could call out over JNI to C++ to do the heavy lifting so I would learn C++ to the point I can do something useful in it.
In terms of mastering C++ over Java or vice versa: learn both to the point you can pass an interview, there are going to be platforms out there that have Java, mastery will come with use.
