public ReaderContextBuilder withFileSystem(FileSystem fs) {
    if (!(fs instanceof HFileSystem)) {
      this.hfs = new HFileSystem(fs);
    } else {
      this.hfs = (HFileSystem) fs;
    }
    return this;
  }
A builder that helps in building up the ReaderContext
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/ReaderContextBuilder.java