Design n URL shortener

// http://www.geeksforgeeks.org/how-to-design-a-tiny-url-or-url-shortener/
// http://stackoverflow.com/questions/742013/how-to-code-a-url-shortener
// https://github.com/delight-im/ShortURL/blob/master/Java/ShortURL.java

How to design a system that takes big URLs like “http://www.geeksforgeeks.org/count-sum-of-digits-in-numbers-from-1-to-n/” and 
converts them into a short 6 character URL. It is given that URLs are stored in database and every URL has an associated integer id.

public class ShortURL {

    public static final String ALPHABET = "23456789bcdfghjkmnpqrstvwxyzBCDFGHJKLMNPQRSTVWXYZ-_";
    public static final int BASE = ALPHABET.length();

    public static String encode(int num) {
        StringBuilder str = new StringBuilder();
        while (num > 0) {
            str.insert(0, ALPHABET.charAt(num % BASE));
            num = num / BASE;
        }
        return str.toString();
    }

    public static int decode(String str) {
        int num = 0;
        for (int i = 0; i < str.length(); i++) {
            num = num * BASE + ALPHABET.indexOf(str.charAt(i));
        }
        return num;
    }

}