/*
 * ShortURL (https://github.com/delight-im/ShortURL)
 * Copyright (c) delight.im (https://www.delight.im/)
 * Licensed under the MIT License (https://opensource.org/licenses/MIT)
 */

/**
 * ShortURL: Bijective conversion between natural numbers (IDs) and short strings
 *
 * ShortURL.encode() takes an ID and turns it into a short string
 * ShortURL.decode() takes a short string and turns it into an ID
 *
 * Features:
 * + large alphabet (51 chars) and thus very short resulting strings
 * + proof against offensive words (removed 'a', 'e', 'i', 'o' and 'u')
 * + unambiguous (removed 'I', 'l', '1', 'O' and '0')
 *
 * Example output:
 * 123456789 <=> pgK8p
 */
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;
	}

	private ShortURL() {}

}

/*
 * ShortURL (https://github.com/delight-im/ShortURL)
 * Copyright (c) delight.im (https://www.delight.im/)
 * Licensed under the MIT License (https://opensource.org/licenses/MIT)
 */

/**
 * ShortURL: Bijective conversion between natural numbers (IDs) and short strings
 *
 * ShortURL.encode() takes an ID and turns it into a short string
 * ShortURL.decode() takes a short string and turns it into an ID
 *
 * Features:
 * + large alphabet (51 chars) and thus very short resulting strings
 * + proof against offensive words (removed 'a', 'e', 'i', 'o' and 'u')
 * + unambiguous (removed 'I', 'l', '1', 'O' and '0')
 *
 * Example output:
 * 123456789 <=> pgK8p
 */
public final 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;
	}

	private ShortURL() {}

}

Make 'ShortURL' class 'final' in Java