Sure, certain, positive, cocksure mean having or showing no doubt.
Sure and certain are often interchangeable. But sure frequently emphasizes the mere subjective state of assurance; certain often suggests more strongly a conviction that is based on definite grounds or on indubitable evidence.
Positive often suggests overconfidence or dogmatism, but it implies conviction or full confidence—in the rightness or correctness of one’s statement or conclusion.
Cocksure tends to carry a strong implication of presumption or overconfidence in positiveness.