JavaAlgorithms/Conversions/AnyBaseToAnyBase.java

package Conversions;

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;

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String n;
        int b1, b2;
        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;
    }
}