Encoded a 10 max length String with up to 64 different characters (using 6 bits per character) in a long type (64 bits).

MiniString.java

/**
 * Encoding a string into a long.
 * <br/>
 * Squeeze a String inside a long type, although we need to accept some limitations.
 * <br/>
 * The long type has 64 bits, so using a base 64 encoding (6 bits) we can store up to 10 characters inside.
 * <br/>
 * We assume that the target String has a maximum length of 10 characters and they are composed only 
 * of uppercase letters, numbers and a limited number of special characters.
 */
public class MiniString {

	long raw;

	public MiniString(String str) {
		raw = encode(str);
	}

	public String get() {
		return decode(raw);
	}

	public static final int MAX_MINISTR_LEN = 10;
	public static final int MINI_STR_BASE = 64;
	public static final String letters = "=ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 _-!?.$&%@#:[]{}()*<>:;',/^";

	public static long encode(String str) {
		String prepared = prepareString(str);
		long encoded = 0;
		for (char c : prepared.toCharArray()) {
			int x = letters.indexOf(c);
			encoded = encoded * MINI_STR_BASE + x;
		}
		return encoded;
	}

	private static String prepareString(String str) {
		StringBuilder prepared = new StringBuilder();
		for (char c : str.toCharArray()) {
			if (letters.indexOf(c) >= 0)
				prepared.append(c);
			if (prepared.length() > MAX_MINISTR_LEN)
				break;
		}
		return prepared.toString();
	}

	public static String decode(long number) {
		StringBuilder decoded = new StringBuilder();
		long remaining = number;

		while (true) {
			int mod = (int) (remaining % MINI_STR_BASE);
			char c = letters.charAt(mod);
			decoded.insert(0, c);
			if (remaining < MINI_STR_BASE)
				break;
			remaining = remaining / MINI_STR_BASE;
		}

		return decoded.toString();
	}
}