src/third_party/protobuf-3/javanano/src/main/java/com/google/protobuf/nano/MapFactories.java

// Protocol Buffers - Google's data interchange format
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package com.google.protobuf.nano;

import java.util.HashMap;
import java.util.Map;

/**
 * Utility class for maps support.
 */
public final class MapFactories {
  public static interface MapFactory {
    <K, V> Map<K, V> forMap(Map<K, V> oldMap);
  }

  // NOTE(liujisi): The factory setter is temporarily marked as package private.
  // The way to provide customized implementations of maps for different
  // platforms are still under discussion.  Mark it as private to avoid exposing
  // the API in proto3 alpha release.
  /* public */ static void setMapFactory(MapFactory newMapFactory) {
    mapFactory = newMapFactory;
  }

  public static MapFactory getMapFactory() {
    return mapFactory;
  }

  private static class DefaultMapFactory implements MapFactory {
    public <K, V> Map<K, V> forMap(Map<K, V> oldMap) {
      if (oldMap == null) {
        return new HashMap<K, V>();
      }
      return oldMap;
    }
  }
  private static volatile MapFactory mapFactory = new DefaultMapFactory();

  private MapFactories() {}
}