BaseConvert.java

import java.math.BigInteger;
import java.io.File;
import java.io.FileOutputStream;

public class BaseConvert
{
  public static void main(String[] args) throws Throwable
	{
		if(args.length < 3)
		{
			java.util.Scanner in = new java.util.Scanner(System.in);
			System.out.println(ConvertBase(in.next(), (char)in.nextInt(), (char)in.nextInt()));
		}
		else if(args.length == 3)
		{
			System.out.println(ConvertBase(args[0], Integer.parseInt(args[1]), Integer.parseInt(args[2])));
		}
		else if(args.length == 4)
		{
			String s = ConvertBase(args[0], Integer.parseInt(args[1]), Integer.parseInt(args[2]));
			File f = new File(args[4]);
			if(!f.exists())
			{
				f.createNewFile();
			}
			FileOutputStream ofs = new FileOutputStream(f);
			ofs.write(s.getBytes());
			ofs.flush();
			ofs.close();
		}
	}
	static String ConvertBase(String num, final int from, final int to)
	{
		if(from < 2 || from > 62 || to < 2 || to > 62) return null;
		final BigInteger bi_from = new BigInteger(""+from);
		final BigInteger bi_to = new BigInteger(""+to);
		BigInteger n = BigInteger.ZERO;
		String sign = "";
		if(num.charAt(0) == '-')
		{
			sign = "-";
			num = num.substring(1);
		}
		int dig = 0;
		for(char c : new StringBuilder(num).reverse().toString().toCharArray())
		{
			int d;
			if(c >= '0' && c <= '9')
			{
				d = c-'0';
			}
			else if(c >= 'A' && c <= 'Z')
			{
				d = c-'A'+10;
			}
			else if(c >= 'a' && c <= 'z')
			{
				d = c-'a'+36;
			}
			else return null;
			n = n.add(bi_from.pow(dig++).multiply(new BigInteger(""+d)));
		}
		StringBuilder sb = new StringBuilder();
		while(!n.equals(BigInteger.ZERO))
		{
			int d = n.mod(bi_to).intValue();
			if(d >= 0 && d <= 9)
			{
				d = '0'+d;
			}
			else if(d >= 10 && d <= 35)
			{
				d = 'A'+d-10;
			}
			else
			{
				d = 'a'+d-36;
			}
			sb.append((char)d);
			n = n.divide(bi_to);
		}
		return sign+sb.reverse();
	}
}