Learn one technology very good. Off course in principle all programming languages are based on the same idea, and
you should understand for example the idea of OOP through Java, which could benefit you programming any OOP language. But, if you are programming, for being up speed, you should know the details, pit falls and bug traps of that one technology. Otherwise your boss complains that you are slow. Since he understands nothing of technology, he will not understand your problem. And if you work like hell and won't chat with your team mates, and not go to the company outing because you have enough to do at home too, he will blame your social skills for everything, and then you're hung. I've been there, I've programmed in C++, Pascal, C#, Java, Visual Basic in one year for one employer, and he then complained..I can still ...ah better not write that down.
But while you're programming in one language, try to get knowledge of general aspects you can use in all environments like OOP,
Unit Testing, regex, Design
Patterns. Then if you loose your java job, and apply for a C# job, you can still manage.