Most likely not.
A certificate would just prove that you know Java, but not how to use it beyond the basics.
Building Java programs and being able to show them off to prospective employers (via a website or something) is going to help you more.
This is, of course, my opinion
A cert would help if you want to become a Java programmer, as I do. The problem is that I have a lot of Java experience in personal projects, but not much professional experience. Having a Java cert at least proves you know the basics. But it is not a substitute for experience.