@Override
  public ImmutableCollection<HStoreFile> clearFiles() {
    ImmutableCollection<HStoreFile> result = state.allFilesCached;
    this.state = new State();
    this.fileStarts.clear();
    this.fileEnds.clear();
    return result;
  }
Normally invalid key is null, but in the map null is the result for "no key"; so use the
following constant value in these maps instead. Note that this is a constant and we use it to
compare by reference when we read from the map.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/StripeStoreFileManager.java