/** 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));

in.close();

}

/** 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;

}

/**

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.

if (0 == decimalValue) return "0";

// 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;

}