public T get(K key) {
    return Optional.ofNullable(memoryMap.get(key))
        .orElse(diskBasedMap.map(diskMap -> diskMap.get(key)).orElse(null));
  }
Approximate memory footprint of the in-memory map.
amoro-format-iceberg/src/main/java/org/apache/amoro/utils/map/SimpleSpillableMap.java