import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

class Ideone
{
	static Map<Integer,Object> map = new ConcurrentHashMap<>();
	static int sz;
	static
	{
		List<Integer> list = Arrays.asList(6, 1, 516, 15, 435, 452, 8, 23, 18, 23, 76, 266, 544, 815, 315, 672, 145, 414);
        sz = list.size();
        for(Integer i : list)
            map.put(i, new Object());
	}
	public static void main (String[] args) throws java.lang.Exception
	{	
        int loop = 1;
        for(Integer i : map.keySet())
        {
            System.out.println("--> "+i+" "+(loop++)+"/"+sz);
            update(i);
        }
        assert(loop == sz);
	}
	static void update(Integer key)
    {
        map.remove(key);
        map.put(key, new Object());
    }
}