Port of Firebase Pushid generator in Java. See original at https://gist.github.com/mikelehen/3596a30bd69384624c11#file-generate-pushid-js

FireBasePushIdGenerator.java

package com.firebase.utils;

import java.util.Date;

/**
 * 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).
 * 
 * @author jfbyers@about.me 
 * @see <a href="https://gist.github.com/mikelehen/3596a30bd69384624c11#file-generate-pushid-js">
   Original port by mikelehen </a> 
 */
public class FireBasePushIdGenerator {
	// Modeled after base64 web-safe chars, but ordered by ASCII.
	private final static String PUSH_CHARS = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";

	public static String generatePushId() {
		// Timestamp of last push, used to prevent local collisions if you push twice in one ms.
		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.
		char[] lastRandChars = new char[72];

		long now = new Date().getTime();

		boolean duplicateTime = (now == lastPushTime);

		char[] timeStampChars = new char[8];
		for (int i = 7; i >= 0; i--) {
			final long module = now % 64;
			timeStampChars[i] = PUSH_CHARS.charAt(Long.valueOf(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] = (char) Math.floor(Double.valueOf(times).intValue());

			}
		} else {
			// If the timestamp hasn't changed since last push, use the same random number,
 			//except incremented by 1.
			int lastValueOfInt=0;
			for (int i = 11; i >= 0 && lastRandChars[i] == 63; i--) {
				lastValueOfInt = i;
				lastRandChars[i] = 0;
			}
			lastRandChars[lastValueOfInt]++;
		}
		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;
	};
	
}

I've made a Java port that works without Date and with minimal object creation.
https://gist.github.com/pepijntb/067db689459fd26949dc

@jfbyers Don't you think duplicateTime will be always false? As lastPushTime is not global variable and is set to zero, and now is current timestamp which is always non zero.

@kalpeshp0310 I agree. It should be outside the method. It should be the member of Class and not the method. Else it will always be zero.

This doesn't work, some variables must be static, else will always fail some conditions. Also it is not very efficient using += with Strings as it will create a new String for every character added.

The lastValueOfInt doesn't work properly

I have added new, correctly working version to this gist: https://gist.github.com/swftvsn/438b4ed68619ad1f5d1c251dc3a5af6f that also contains a junit test to verify multithreaded operation.

@aokholm is correct, lastRandChars should have length of 12.

@abeatte I think you misunderstood where 72 bits comes from: 12 chars of lastRandChars is filled with random value of 0-64 which is 6 bits.