Dfs.CacheEntry cacheEntry = new Dfs.CacheEntry(ttl);
        long expectedExpiration = System.currentTimeMillis() + ttl * 1000L;
        assertTrue(cacheEntry.expiration >= expectedExpiration - 100 && cacheEntry.expiration <= expectedExpiration + 100);
        assertNotNull(cacheEntry.map);

        // Test with TTL = 0, should use default TTL
        Dfs.CacheEntry cacheEntryDefault = new Dfs.CacheEntry(0);

  @Nullable V getIfCached(@Nullable Object key) {
    Entry<K, V> entry = cacheEntry; // store local reference for thread-safety

    // Check cache. We use == on purpose because it's cheaper and a cache miss is ok.
    if (entry != null && entry.getKey() == key) {
      return entry.getValue();
    }
    return null;
  }

  void clearCache() {
    cacheEntry = null;
  }

  }

  private void addToCache(K key, V value) {
    addToCache(new CacheEntry<K, V>(key, value));
  }

  private void addToCache(CacheEntry<K, V> entry) {
    // Slide new entry into first cache position. Drop previous entry in second cache position.
    cacheEntry2 = cacheEntry1;
    cacheEntry1 = entry;
  }

  private static final class CacheEntry<K, V> {
    final K key;
    final V value;