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diff --git a/java/src/main/java/com/google/protobuf/MapField.java b/java/src/main/java/com/google/protobuf/MapField.java
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+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+package com.google.protobuf;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Internal representation of map fields in generated messages.
+ * 
+ * This class supports accessing the map field as a {@link Map} to be used in
+ * generated API and also supports accessing the field as a {@link List} to be
+ * used in reflection API. It keeps track of where the data is currently stored
+ * and do necessary conversions between map and list.  
+ * 
+ * This class is a protobuf implementation detail. Users shouldn't use this
+ * class directly.
+ * 
+ * THREAD-SAFETY NOTE: Read-only access is thread-safe. Users can call getMap()
+ * and getList() concurrently in multiple threads. If write-access is needed,
+ * all access must be synchronized.
+ */
+public class MapField<K, V> {
+  /**
+   * Indicates where the data of this map field is currently stored.
+   * 
+   * MAP: Data is stored in mapData.
+   * LIST: Data is stored in listData.
+   * BOTH: mapData and listData have the same data.
+   *
+   * When the map field is accessed (through generated API or reflection API),
+   * it will shift between these 3 modes:
+   * 
+   *          getMap()   getList()   getMutableMap()   getMutableList()
+   *   MAP      MAP        BOTH          MAP               LIST
+   *   LIST     BOTH       LIST          MAP               LIST
+   *   BOTH     BOTH       BOTH          MAP               LIST
+   *   
+   * As the map field changes its mode, the list/map reference returned in a
+   * previous method call may be invalidated. 
+   */
+  private enum StorageMode {MAP, LIST, BOTH}
+
+  private volatile StorageMode mode;
+  private Map<K, V> mapData;
+  private List<Message> listData;
+  
+  // Convert between a map entry Message and a key-value pair.
+  private static interface Converter<K, V> {
+    Message convertKeyAndValueToMessage(K key, V value);
+    void convertMessageToKeyAndValue(Message message, Map<K, V> map);
+    
+    Message getMessageDefaultInstance();
+  }
+  
+  private static class ImmutableMessageConverter<K, V> implements Converter<K, V> {
+    private final MapEntry<K, V> defaultEntry;
+    public ImmutableMessageConverter(MapEntry<K, V> defaultEntry) {
+      this.defaultEntry = defaultEntry;
+    }
+    
+    public Message convertKeyAndValueToMessage(K key, V value) {
+      return defaultEntry.newBuilderForType().setKey(key).setValue(value).build();
+    }
+    
+    public void convertMessageToKeyAndValue(Message message, Map<K, V> map) {
+      MapEntry<K, V> entry = (MapEntry<K, V>) message;
+      map.put(entry.getKey(), entry.getValue());
+    }
+
+    public Message getMessageDefaultInstance() {
+      return defaultEntry;
+    }
+  }
+  
+
+  private final Converter<K, V> converter;
+  
+  private MapField(
+      Converter<K, V> converter,
+      StorageMode mode,
+      Map<K, V> mapData,
+      List<Message> listData) {
+    this.converter = converter;
+    this.mode = mode;
+    this.mapData = mapData;
+    this.listData = listData;
+  }
+    
+  private MapField(
+      MapEntry<K, V> defaultEntry,
+      StorageMode mode,
+      Map<K, V> mapData,
+      List<Message> listData) {
+    this(new ImmutableMessageConverter<K, V>(defaultEntry), mode, mapData, listData);
+  }
+  
+  
+  /** Returns an immutable empty MapField. */
+  public static <K, V> MapField<K, V> emptyMapField(
+      MapEntry<K, V> defaultEntry) {
+    return new MapField<K, V>(
+        defaultEntry, StorageMode.MAP, Collections.<K, V>emptyMap(), null);
+  }
+  
+  
+  /** Creates a new mutable empty MapField. */
+  public static <K, V> MapField<K, V> newMapField(MapEntry<K, V> defaultEntry) {
+    return new MapField<K, V>(
+        defaultEntry, StorageMode.MAP, new HashMap<K, V>(), null);
+  }
+  
+  
+  private Message convertKeyAndValueToMessage(K key, V value) {
+    return converter.convertKeyAndValueToMessage(key, value);
+  }
+  
+  @SuppressWarnings("unchecked")
+  private void convertMessageToKeyAndValue(Message message, Map<K, V> map) {
+    converter.convertMessageToKeyAndValue(message, map);
+  }
+
+  private List<Message> convertMapToList(Map<K, V> mapData) {
+    List<Message> listData = new ArrayList<Message>();
+    for (Map.Entry<K, V> entry : mapData.entrySet()) {
+      listData.add(
+          convertKeyAndValueToMessage(
+              entry.getKey(), entry.getValue()));
+    }
+    return listData;
+  }
+
+  private Map<K, V> convertListToMap(List<Message> listData) {
+    Map<K, V> mapData = new HashMap<K, V>();
+    for (Message item : listData) {
+      convertMessageToKeyAndValue(item, mapData);
+    }
+    return mapData;
+  }
+  
+  /** Returns the content of this MapField as a read-only Map. */
+  public Map<K, V> getMap() {
+    if (mode == StorageMode.LIST) {
+      synchronized (this) {
+        if (mode == StorageMode.LIST) {
+          mapData = convertListToMap(listData);
+          mode = StorageMode.BOTH;
+        }
+      }
+    }
+    return Collections.unmodifiableMap(mapData);
+  }
+  
+  /** Gets a mutable Map view of this MapField. */
+  public Map<K, V> getMutableMap() {
+    if (mode != StorageMode.MAP) {
+      if (mode == StorageMode.LIST) {
+        mapData = convertListToMap(listData);
+      }
+      listData = null;
+      mode = StorageMode.MAP; 
+    }
+    return mapData;
+  }
+  
+  public void mergeFrom(MapField<K, V> other) {
+    getMutableMap().putAll(MapFieldLite.copy(other.getMap()));
+  }
+  
+  public void clear() {
+    mapData = new HashMap<K, V>();
+    mode = StorageMode.MAP;
+  }
+  
+  @SuppressWarnings("unchecked")
+  @Override
+  public boolean equals(Object object) {
+    if (!(object instanceof MapField)) {
+      return false;
+    }
+    MapField<K, V> other = (MapField<K, V>) object;
+    return MapFieldLite.<K, V>equals(getMap(), other.getMap());
+  }
+  
+  @Override
+  public int hashCode() {
+    return MapFieldLite.<K, V>calculateHashCodeForMap(getMap());
+  }
+  
+  /** Returns a deep copy of this MapField. */
+  public MapField<K, V> copy() {
+    return new MapField<K, V>(
+        converter, StorageMode.MAP, MapFieldLite.copy(getMap()), null);
+  }
+  
+  /** Gets the content of this MapField as a read-only List. */
+  List<Message> getList() {
+    if (mode == StorageMode.MAP) {
+      synchronized (this) {
+        if (mode == StorageMode.MAP) {
+          listData = convertMapToList(mapData);
+          mode = StorageMode.BOTH;
+        }
+      }
+    }
+    return Collections.unmodifiableList(listData);
+  }
+  
+  /** Gets a mutable List view of this MapField. */
+  List<Message> getMutableList() {
+    if (mode != StorageMode.LIST) {
+      if (mode == StorageMode.MAP) {
+        listData = convertMapToList(mapData);
+      }
+      mapData = null;
+      mode = StorageMode.LIST;
+    }
+    return listData;
+  }
+  
+  /**
+   * Gets the default instance of the message stored in the list view of this
+   * map field.
+   */
+  Message getMapEntryMessageDefaultInstance() {
+    return converter.getMessageDefaultInstance();
+  }
+}