AnyBaseToAnyBase.java

import java.util.Arrays;
import java.util.HashSet;
import java.util.InputMismatchException;
import java.util.Scanner;

/**
 * Class for converting from "any" base to "any" other base, when "any" means from 2-36.
 * Works by going from base 1 to decimal to base 2. Includes auxiliary method for 
 * determining whether a number is valid for a given base.
 * 
 * @author Michael Rolland
 * @version 2017.10.10
 *
 */
public class AnyBaseToAnyBase {
	
	// Smallest and largest base you want to accept as valid input
	static final int MINIMUM_BASE = 2;
	static final int MAXIMUM_BASE = 36;
	
	// Driver
	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		String n;
		int b1=0,b2=0;
		while (true) {
			try {
				System.out.print("Enter number: ");
				n = in.next();
				System.out.print("Enter beginning base (between "+MINIMUM_BASE+" and "+MAXIMUM_BASE+"): ");
				b1 = in.nextInt();
				if (b1 > MAXIMUM_BASE || b1 < MINIMUM_BASE) {
					System.out.println("Invalid base!");
					continue;
				}
				if (!validForBase(n, b1)) {
					System.out.println("The number is invalid for this base!");
					continue;
				}
				System.out.print("Enter end base (between "+MINIMUM_BASE+" and "+MAXIMUM_BASE+"): ");
				b2 = in.nextInt();
				if (b2 > MAXIMUM_BASE || b2 < MINIMUM_BASE) {
					System.out.println("Invalid base!");
					continue;
				}
				break;
			} catch (InputMismatchException e) {
				System.out.println("Invalid input.");
				in.next();
			}
		}
		System.out.println(base2base(n, b1, b2));
	}
	
	/**
	 * Checks if a number (as a String) is valid for a given base.
	 */
	public static boolean validForBase(String n, int base) {
		char[] validDigits = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E',
				'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V',
				'W', 'X', 'Y', 'Z'};
		// digitsForBase contains all the valid digits for the base given
		char[] digitsForBase = Arrays.copyOfRange(validDigits, 0, base);
		
		// Convert character array into set for convenience of contains() method
		HashSet<Character> digitsList = new HashSet();
		for (int i=0; i<digitsForBase.length; i++)
			digitsList.add(digitsForBase[i]);
		
		// Check that every digit in n is within the list of valid digits for that base.
		for (char c : n.toCharArray()) 
			if (!digitsList.contains(c))
				return false;
		
		return true;
	}
	
	/**
	 * Method to convert any integer from base b1 to base b2. Works by converting from b1 to decimal,
	 * then decimal to b2.
	 * @param n The integer to be converted.
	 * @param b1 Beginning base.
	 * @param b2 End base.
	 * @return n in base b2.
	 */
	public static String base2base(String n, int b1, int b2) {
		// Declare variables: decimal value of n, 
		// character of base b1, character of base b2,
		// and the string that will be returned.
		int decimalValue = 0, charB2;
		char charB1;
		String output="";
		// Go through every character of n
		for (int i=0; i<n.length(); i++) {
			// store the character in charB1
			charB1 = n.charAt(i);
			// if it is a non-number, convert it to a decimal value >9 and store it in charB2
			if (charB1 >= 'A' && charB1 <= 'Z') 
				charB2 = 10 + (charB1 - 'A');
			// Else, store the integer value in charB2
			else 
				charB2 = charB1 - '0';
			// Convert the digit to decimal and add it to the
			// decimalValue of n
			decimalValue = decimalValue * b1 + charB2;
		}
		
		// Converting the decimal value to base b2:
		// A number is converted from decimal to another base
		// by continuously dividing by the base and recording 
		// the remainder until the quotient is zero. The number in the
		// new base is the remainders, with the last remainder
		// being the left-most digit.
		
		// While the quotient is NOT zero:
		while (decimalValue != 0) {
			// If the remainder is a digit < 10, simply add it to
			// the left side of the new number.
			if (decimalValue % b2 < 10) 
				output = Integer.toString(decimalValue % b2) + output;
			// If the remainder is >= 10, add a character with the
			// corresponding value to the new number. (A = 10, B = 11, C = 12, ...)
			else
				output = (char)((decimalValue % b2)+55) + output;
			// Divide by the new base again
			decimalValue /= b2;
		}
		return output;
	}
}