You've got it the other way around. That's like saying I can put a Post-It note on my Toyota Corolla that says "This is a Porsche" and my Corolla becomes a Porsche. It doesn't work like that. You don't achieve faster access just by implementing the RandomAccess marker interface.
Rather, and this is explained in the API documentation for java.util.RandomAccess, if your implementation is such that for typical instances of the implementation class, you'd get better performance by using get(i) than you would by using an Iterator and its next() method, then you should mark your class as implementing RandomAccess.
Extending the analogy I gave earlier, if you actually do have a Porsche, then it should have a Porsche emblem somewhere on it so people who don't know much about cars can at least see the emblem and say, "Oh, that's a Porsche."