### Question:
Counters extends AbstractCounters<Counters.Counter, Counters.Group> { public void incrCounter(Enum<?> key, long amount) { findCounter(key).increment(amount); } Counters();  Counters(org.apache.hadoop.mapreduce.Counters newCounters); synchronized Group getGroup(String groupName); @SuppressWarnings("unchecked") synchronized Collection<String> getGroupNames(); synchronized String makeCompactString(); synchronized Counter findCounter(String group, String name); @Deprecated Counter findCounter(String group, int id, String name); void incrCounter(Enum<?> key, long amount); void incrCounter(String group, String counter, long amount); synchronized long getCounter(Enum<?> key); synchronized void incrAllCounters(Counters other); int size(); static Counters sum(Counters a, Counters b); void log(Log log); String makeEscapedCompactString(); static Counters fromEscapedCompactString(String compactString); static int MAX_COUNTER_LIMIT; }### Answer:
@SuppressWarnings("deprecation") @Test public void testCounterIteratorConcurrency() { Counters counters = new Counters(); counters.incrCounter("group1", "counter1", 1); Iterator<Group> iterator = counters.iterator(); counters.incrCounter("group2", "counter2", 1); iterator.next(); }