* intermediate zero-state. Ways we could deal with this are:
   *
   * - Don't define any of the ConcurrentMap operations. This is the current state of affairs.
   *
   * - Define putIfAbsent and replace as treating zero and absent identically (as currently
   *   implemented below). This is a bit surprising with putIfAbsent, which really becomes
   *   putIfZero.

    // since get returned null, try a putIfAbsent; that fails due to a simulated race
    when(backingMap.putIfAbsent(eq(KEY), isA(AtomicInteger.class))).thenReturn(existingZero);
    // since the putIfAbsent returned a zero, we'll try to replace...
    when(backingMap.replace(eq(KEY), eq(existingZero), isA(AtomicInteger.class))).thenReturn(false);
    // ...and then putIfAbsent. Simulate failure on both

        Entry<Object, Object> entry = warmed.get(i - WARMUP_MIN);
        Object newValue = new Object();
        assertThat(cache.asMap().putIfAbsent(entry.getKey(), newValue))
            .isSameInstanceAs(entry.getValue());
        Object newKey = new Object();
        assertThat(cache.asMap().putIfAbsent(newKey, entry.getValue())).isNull();
        // this getUnchecked() call shouldn't be a cache miss; verified below

              entry.getKey(), entry.getValue().getValue(), RemovalCause.EXPLICIT);
        }
      }
      cachingHashMap.clear();
    }
  }

  @Override
  public @Nullable V putIfAbsent(K key, V value) {
    synchronized (lock) {
      V currentValue = get(key);
      if (currentValue != null) {
        return currentValue;
      }
      return put(key, value);
    }
  }

    for (int i = 0; i < ITERATIONS; i++) {
      long before = map.get(key);
      long result = map.putIfAbsent(key, newValue);
      long after = map.get(key);
      assertEquals(before, result);
      assertEquals(before == 0 ? newValue : before, after);

      map.remove(key);
      before = map.get(key);
      result = map.putIfAbsent(key, newValue);
      after = map.get(key);
      assertEquals(0, before);

    }
    checkPositive(occurrences, "occurrences");

    while (true) {
      AtomicInteger existingCounter = safeGet(countMap, element);
      if (existingCounter == null) {
        existingCounter = countMap.putIfAbsent(element, new AtomicInteger(occurrences));
        if (existingCounter == null) {
          return 0;
        }
        // existingCounter != null: fall through to operate against the existing AtomicInteger
      }

    map.putAll(ImmutableMap.of(two, three));
    assertThat(map.get(two)).isSameInstanceAs(three);
    assertThat(map.putIfAbsent(one, three)).isSameInstanceAs(two);
    assertThat(map.get(one)).isSameInstanceAs(two);
    assertThat(map.putIfAbsent(three, one)).isNull();
    assertThat(map.get(three)).isSameInstanceAs(one);
    assertThat(map.replace(one, three)).isSameInstanceAs(two);

    map.putAll(ImmutableMap.of(two, three));
    assertThat(map.get(two)).isSameInstanceAs(three);
    assertThat(map.putIfAbsent(one, three)).isSameInstanceAs(two);
    assertThat(map.get(one)).isSameInstanceAs(two);
    assertThat(map.putIfAbsent(three, one)).isNull();
    assertThat(map.get(three)).isSameInstanceAs(one);
    assertThat(map.replace(one, three)).isSameInstanceAs(two);

    }
    checkPositive(occurrences, "occurrences");

    while (true) {
      AtomicInteger existingCounter = safeGet(countMap, element);
      if (existingCounter == null) {
        existingCounter = countMap.putIfAbsent(element, new AtomicInteger(occurrences));
        if (existingCounter == null) {
          return 0;
        }
        // existingCounter != null: fall through to operate against the existing AtomicInteger
      }

    for (int i = 0; i < ITERATIONS; i++) {
      long before = map.get(key);
      long result = map.putIfAbsent(key, newValue);
      long after = map.get(key);
      assertEquals(before, result);
      assertEquals(before == 0 ? newValue : before, after);

      map.remove(key);
      before = map.get(key);
      result = map.putIfAbsent(key, newValue);
      after = map.get(key);
      assertEquals(0, before);