To check race condition about ConcurrentHashMap. if you use ConcurrentHashMap, you should atomic method. (e.g, putIfAbsent)

ConcurrentTest.java

import java.util.concurrent.ConcurrentHashMap;
public class ConcurrentTest {
	private static ConcurrentHashMap<Integer, Integer> map = new ConcurrentHashMap<Integer, Integer>();

	private static class Update extends Thread {
		@Override
		public void run() {
			for (int i = 0; i < 100; i++) {
				// forrowing code is incorrect. (probabry happens race condition) 
				if (!map.contains(i)) {
					map.put(i, i);
					System.out.println("puted [" + i + "] by " + getName());
				}
				// fix to map.putIfAbsent(i, i);
			}
		}
	}
	public static void main(String[] args) {
		Update t1 = new Update();
		Update t2 = new Update();
		t1.start();
		t2.start();
	}
}