content: // 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;
  
  // TODO(chrisn): Change ContainingType to extend Message
  /**
   * Interface that generated extensions implement.
   *
   * @author liujisi@google.com (Jisi Liu)
   */
  public abstract class Extension<ContainingType extends MessageLite, Type>
      extends ExtensionLite<ContainingType, Type> {
    // TODO(chrisn): Add package-private constructor.
  
    /** {@inheritDoc} Overridden to return {@link Message} instead of {@link MessageLite}. */
    @Override
    public abstract Message getMessageDefaultInstance();
  
    /** Returns the descriptor of the extension. */
    public abstract Descriptors.FieldDescriptor getDescriptor();
  
    /** Returns whether or not this extension is a Lite Extension. */
    @Override
    final boolean isLite() {
      return false;
    }
  
    // All the methods below are extension implementation details.
  
    /** The API type that the extension is used for. */
    protected enum ExtensionType {
      IMMUTABLE,
      MUTABLE,
      PROTO1,
    }
  
    protected abstract ExtensionType getExtensionType();
  
    /** Type of a message extension. */
    public enum MessageType {
      PROTO1,
      PROTO2,
    }
  
    /**
     * If the extension is a message extension (i.e., getLiteType() == MESSAGE), returns the type of
     * the message, otherwise undefined.
     */
    public MessageType getMessageType() {
      return MessageType.PROTO2;
    }
  
    protected abstract Object fromReflectionType(Object value);
  
    protected abstract Object singularFromReflectionType(Object value);
  
    protected abstract Object toReflectionType(Object value);
  
    protected abstract Object singularToReflectionType(Object value);
  }
  