[Java] ArrayListを複数スレッドから使用した場合に発生する競合のテスト

TestRaceConditionOfArrayList.java

public class TestRaceConditionOfArrayList {
  private static final int NUMBER_OF_TRIALS = 1_000_000;
  private static final int NUMBER_OF_THREADS = 8;
  private static final List<Integer> SOURCE_LIST = Arrays.asList(1, 2);

  public static void main(String[] args) throws InterruptedException, ExecutionException {
    ExecutorService executor = Executors.newFixedThreadPool(NUMBER_OF_THREADS);
    List<Future<String>> futures = new ArrayList<>();
    Testee testee = new Testee();
    for (int i = 0; i < NUMBER_OF_TRIALS; i++) {
      futures.add(executor.submit(testee));
    }
    print(count(futures));
    executor.shutdown();
  }

  static class Testee implements Callable<String> {
    ArrayList<Integer> testeeList = new ArrayList<>();

    @Override
    public String call() {
      testeeList = new ArrayList<>();
      for (int item : SOURCE_LIST) {
        testeeList.add(item);
      }
      return new ArrayList<>(testeeList).toString();
    }
  }

  static Map<String, Integer> count(List<Future<String>> futures)
      throws InterruptedException, ExecutionException {
    Map<String, Integer> countMap = new HashMap<>();
    for (Future<String> res : futures) {
      String key = res.get();
      countMap.put(key, countMap.containsKey(key) ? countMap.get(key) + 1 : 1);
    }
    return countMap;
  }

  static void print(Map<String, Integer> countMap) {
    List<Entry<String, Integer>> sortedEntries = sort(countMap);
    for (Entry<String, Integer> item : sortedEntries) {
      System.out.println(item);
    }
  }

  static List<Entry<String, Integer>> sort(Map<String, Integer> countMap) {
    List<Entry<String, Integer>> res = new ArrayList<>(countMap.entrySet());
    Collections.sort(res, new Comparator<Entry<String, Integer>>() {
      public int compare(Entry<String, Integer> obj1, Entry<String, Integer> obj2) {
        return obj2.getValue().compareTo(obj1.getValue()); // descending order of value
      }
    });
    return res;
  }
}