The Java program below does fairly general number base conversions. It doesn't handle bases bigger than 10, though that would be fairly easy to include.

// ConvertBase.java: convert an int n to base b
public class ConvertBase {
   private static char ch; // char location to hold next

   public static String convertToBase(int n, int b) {
      String s = new String();
      int r;
      while (n != 0) {
         r = n%b;
         ch = (char)(r + '0');
         s = ch + s;
         n = n/b;
      }
      return s;
   }

   // main function to try out Base class
   public static void main (String[] args) {
      int n; // number to convert to base b
      int b; // base for number conversions
      while(true) {
         System.out.print("Number to convert ----->");
         n = GetNext.getNextInt();
         if (n == 0) break;
         System.out.print("Base for conversion --->");
         b = GetNext.getNextInt();
         String conv = new String();
         conv = ConvertBase.convertToBase(n, b);
         System.out.println(n + " to base " + b + " = " + conv);
      }
   } // end of main
}  

// GetNext.java: fetch the next char or int from standard input
import java.io.*;
public class GetNext {
   // in: reader for reading input data
   private static Reader in = new InputStreamReader(System.in);

   // ch: holds the next character read
   private static char ch;

   public static char getNextChar() {
      try {
         ch = (char)in.read();
      }
      catch (Exception exception)
      {
         System.out.println("Error reading character");
         ch = ' ';
      }
      return ch;
   }

   public static int getNextInt() {
      int num;  // the number to convert
      int sign = 1; // gives sign of answer
      ch = getNextChar();
      // skip over initial blanks and newlines
      while (ch == ' ' || ch == '\n')
         ch = getNextChar();

      // check for + or - sign
      if (ch == '+') ch = getNextChar();
      if (ch == '-') {
         sign = -1;
         ch = getNextChar();
      }

      // read digits, construct input integer;
      // (same as the method "Integer.parseInt()"  )
      num = 0;
      while (Character.isDigit(ch)) {
         num = num*10 + (ch - '0');
         ch = getNextChar();
      }
      return sign * num;
   }
}

% javac GetNext.java
% javac ConvertBase.java
% java ConvertBase
Number to convert ----->153
Base for conversion --->8
153 to base 8 = 231

Number to convert ----->153
Base for conversion --->2
153 to base 2 = 10011001

Number to convert ----->1000000000
Base for conversion --->2
1000000000 to base 2 = 111011100110101100101000000000

Number to convert ----->1000
Base for conversion --->8
1000 to base 8 = 1750

Number to convert ----->0