@Override
  public void probe(final K key) {
    Preconditions.checkNotNull(key);
    Entry<K, V> oldExclusive;
    synchronized (this) {
      oldExclusive = mExclusiveEntries.remove(key);
      if (oldExclusive != null) {
        mExclusiveEntries.put(key, oldExclusive);
      }
    }
  }
Probes whether the object corresponding to the key is in the cache. Note that the act of
probing touches the item (if present in cache), thus changing its LRU timestamp.
imagepipeline-base/src/main/java/com/facebook/imagepipeline/cache/LruCountingMemoryCache.java