I think you should go for the master's degree.
It's for the long term. It will make you more resilient to changes in computing that will come for sure.
And do it while you're at it. Doing it later will be much harder.
The question is why don't you know Java to your satisfaction after a bachelor's degree? This must be considered a severe flaw of your bachelor's program so maybe you should do your masters elsewhere. But even so, no one with a higher degree needs a Java course to get a job as Java programmer. With any of these degrees, if you have a nack for programming, you will get a job regardless of language. In fact adding courses and certifications in specific langages to an academic degree may even be detremential to your chances of getting a first job!
well, there's no law saying an education in programming or computer science MUST use Java.
There are many languages out there just as suitable for learning the craft.
In fact there are languages specifically designed for the purpose of teaching programming.
And of course an academic environment isn't a place where the development of technical skills applicable to a professional environment should be foremost on the mind.
One can always learn a new programming language after all, just as one can always learn to use a new brand of CNC machine once one has been taught how to use one in general.