Hexadecimal representation came into wide use when the defacto standard for the number of bits in each byte of computer memory became eight which is a multiple of four. Hexadecimal numbers are commonly represented using the following digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F. Note that there are sixteen unique digits in the hexadecimal number system. In Java, the individual letters that are used as digits can be either upper case or lower case.