Julius Caesar likely never knew Japan existed. Julius Caesar likely didn't even know China existed. The Chinese themselves only knew of Rome sometime late in the Han Dynasty, where it was known as "Great Qin."

Japan was also only first mentioned in the late Han Dynasty as a "Kingdom of Yamatai." For instance, the Romance of Three Kingdoms (which is historical fiction about the Three Kingdoms period (~300AD) written in the Ming Dynasty) recalls a conversation where they discuss a Yamatai which has a Queen, Hibiko. Hibiko, for that matter, is considered s semi-mythical ruler of Japan.

Japan as we know it today was not fully organized until the subsequent Yamato period, and was not really well known by China until after Prince Shoutoku's embassies in the Nara period, a good 900 years after Caesar.