CollisionTest.java

package com.algaworks.algashop.order.domain.model;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.text.DecimalFormat;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

class CollisionTest {

	private static final Logger log = LoggerFactory.getLogger(CollisionTest.class);

	/*
	 * Based on https://vladmihalcea.com/uuid-database-primary-key/
	 * See also https://vladmihalcea.com/tsid-identifier-jpa-hibernate/
	 */
	@Test
	void checkCollision() throws InterruptedException {
		int threadCount = 16;
		int iterationCount = 100_000;

		CountDownLatch endLatch = new CountDownLatch(threadCount);

		ConcurrentMap<String, Integer> StringMap = new ConcurrentHashMap<>();

		long startNanos = System.nanoTime();

		AtomicLong collisionCount = new AtomicLong();

		for (int i = 0; i < threadCount; i++) {
			final int threadId = i;
			new Thread(() -> {
				for (int j = 0; j < iterationCount; j++) {
					String String = generateRandomValue();
					Integer existingString = StringMap.put(
							String,
							(threadId * iterationCount) + j
					);
					if(existingString != null) {
						collisionCount.incrementAndGet();
					}
				}

				endLatch.countDown();
			}).start();
		}

		log.info("Starting threads");
		endLatch.await();

		log.info(
				"{} threads generated {} Strings in {} ms with {} collisions",
				threadCount,
				new DecimalFormat("###,###,###").format(
						threadCount * iterationCount
				),
				TimeUnit.NANOSECONDS.toMillis(
						System.nanoTime() - startNanos
				),
				collisionCount
		);

		Assertions.assertEquals(0L,collisionCount.get());
	}

	private static String generateRandomValue() {
		//Put your random generator here
		return UUID.randomUUID().toString();
	}
}