Port of Firebase Push ID generator in Java (ref: https://gist.github.com/mikelehen/3596a30bd69384624c11)

FirebasePushIDGenerator.java

/**
 * Fancy ID generator that creates 20-character string identifiers with the following properties:
 *
 * 1. They're based on timestamp so that they sort *after* any existing ids.
 * 2. They contain 72-bits of random data after the timestamp so that IDs won't collide with other clients' IDs.
 * 3. They sort *lexicographically* (so the timestamp is converted to characters that will sort properly).
 * 4. They're monotonically increasing.  Even if you generate more than one in the same timestamp, the
 *    latter ones will sort after the former ones.  We do this by using the previous random bits
 *    but "incrementing" them by 1 (only in the case of a timestamp collision).
 */
public class FirebasePushIDGenerator {
	// Modeled after base64 web-safe chars, but ordered by ASCII.
	private final static String PUSH_CHARS = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";

	// Timestamp of last push, used to prevent local collisions if you push twice in one ms.
	private static long lastPushTime = 0L;

	// We generate 72-bits of randomness which get turned into 12 characters and appended to the
	// timestamp to prevent collisions with other clients.  We store the last characters we
	// generated because in the event of a collision, we'll use those same characters except
	// "incremented" by one.
	private static int[] lastRandChars = new int[72];

	public static String generatePushId() {
		long now = System.currentTimeMillis();
		boolean duplicateTime = (now == lastPushTime);
		lastPushTime = now;

		char[] timeStampChars = new char[8];
		for (int i = 7; i >= 0; i--) {
			final Long module = now % 64;
			timeStampChars[i] = PUSH_CHARS.charAt(module.intValue());
			now = (long) Math.floor(now / 64);
		}
		if (now != 0) {
			throw new AssertionError("We should have converted the entire timestamp.");
		}

		String id = new String(timeStampChars);

		if (!duplicateTime) {
			for (int i = 0; i < 12; i++) {
				final Double times = Math.random() * 64;
				lastRandChars[i] = (int) Math.floor(times.intValue());
			}
		} else {
			// If the timestamp hasn't changed since last push, use the same random number, except incremented by 1.
			int i = 0;
			for (i = 11; i >= 0 && lastRandChars[i] == 63; i--) {
				lastRandChars[i] = 0;
			}
			lastRandChars[i]++;
		}
		for (int i = 0; i < 12; i++) {
			id += PUSH_CHARS.charAt(lastRandChars[i]);
		}
		if (id.length() != 20) {
			throw new AssertionError("Length should be 20.");
		}

		return id;
	};

}