There are lots of people here who will freely give their time to help you become the best Java programmer you can be. There's nobody here who is interested in helping you cheat or doing your homework for you.
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This is something you can easily do yourself! Just take your numeral one digit at a time from left to right and keep an accumulator that starts at 0. For each digit, multiply the accumulator by the base and then add the value of the digit. You can reverse the process to print out the number in whatever base you like by using % and division, but of course that will get the digits from right to left so you'd have to construct the string before writing it.