@Nullable
    public Integer getValue(E element) {
      final int index = mFirstList.indexOf(element);
      if (index < 0) {
        return null;
      }
      return mSecondList.get(index);
    }
returns the value mapped to the passed element if it exists. Otherwise, returns null.
imagepipeline-base/src/main/java/com/facebook/imagepipeline/cache/AbstractAdaptiveCountingMemoryCache.java