Class to encode a string into base 62 (character set [a-zA-Z0-9]) with Java. A common use case is URL shortening.

Base62.java

public class Base62 {

	public static final String ALPHABET = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

	public static final int BASE = ALPHABET.length();

	private Base62() {}

	public static String fromBase10(int i) {
		StringBuilder sb = new StringBuilder("");
		if (i == 0) {
			return "a";
		}
		while (i > 0) {
			i = fromBase10(i, sb);
		}
		return sb.reverse().toString();
	}

	private static int fromBase10(int i, final StringBuilder sb) {
		int rem = i % BASE;
		sb.append(ALPHABET.charAt(rem));
		return i / BASE;
	}

	public static int toBase10(String str) {
		return toBase10(new StringBuilder(str).reverse().toString().toCharArray());
	}

	private static int toBase10(char[] chars) {
		int n = 0;
		for (int i = chars.length - 1; i >= 0; i--) {
			n += toBase10(ALPHABET.indexOf(chars[i]), i);
		}
		return n;
	}

	private static int toBase10(int n, int pow) {
		return n * (int) Math.pow(BASE, pow);
	}
}

Base62Test.java

import org.junit.Test;
import static junit.framework.Assert.assertEquals;

public class Base62Test {

	@Test
	public void testCharList() throws Exception {
		StringBuilder sb = new StringBuilder();
		for (char c = 'a'; c <= 'z'; c++) {
			sb.append(c);
		}
		for (char c = 'A'; c <= 'Z'; c++) {
			sb.append(c);
		}
		for (int i = 0; i <= 9; i++) {
			sb.append(i);
		}
		assertEquals(sb.toString(), Base62.ALPHABET);
	}

	@Test
	public void testStringFromInt() throws Exception {
		int n = 0;
		String str = "6JaY2";
		char[] chars = str.toCharArray();
		n += Base62.ALPHABET.indexOf(chars[0]) * (int) Math.pow(62, 4);
		n += Base62.ALPHABET.indexOf(chars[1]) * (int) Math.pow(62, 3);
		n += Base62.ALPHABET.indexOf(chars[2]) * (int) Math.pow(62, 2);
		n += Base62.ALPHABET.indexOf(chars[3]) * (int) Math.pow(62, 1);
		n += Base62.ALPHABET.indexOf(chars[4]) * (int) Math.pow(62, 0);
		assertEquals(str, Base62.fromBase10(n));
	}
	
	@Test
	public void testIntegerFromString() throws Exception {
		assertEquals(125, Base62.toBase10("cb"));
	}

	@Test
	public void testFromZero() {
		assertEquals("a", Base62.fromBase10(0));
		assertEquals("b", Base62.fromBase10(1));
	}
}

guidomedina - nice solution!

any string that starts with 'a' is not working

@guidomedina I would like to use your code in SchemaSpy, would it be ok? You'll retain copyright and it will be distributed under the terms of LGPL v3 or later.

nice practice I think that makes more sense start with 0, instead of 'a'

about @guidomedina example above. Be careful as his base62(int) method works with O(n^2) asymptotics, because sb.insert(0, ..) does system.arraycopy under the hood. It's better to do sb.append(...) and reverse the result at the end. We would have O(2n) which is O(n) asymptotically.

* assuming math operations are performed in constant time.

Edited my initial code sample to append and reverse ;-)

@npetzall I'm so sorry I missed your message, feel free, this is just a simple method, feel free to do with it whatever you want/need