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Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. This project includes GWT-friendly sources.
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/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS-IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import com.google.common.annotations.GwtCompatible;

import java.util.Comparator;
import java.util.HashMap;
import java.util.TreeMap;

/**
 * Contains dummy collection implementations to convince GWT that part of
 * serializing a collection is serializing its elements.
 *
 * Because of our use of final fields in our collections, GWT's normal
 * heuristic for determining which classes might be serialized fails. That
 * heuristic is, roughly speaking, to look at each parameter and return type of
 * each RPC interface and to assume that implementations of those types might be
 * serialized. Those types have their own dependencies -- their fields -- which
 * are analyzed recursively and analogously.
 *
 * 
For classes with final fields, GWT assumes that the class itself might be
 * serialized but doesn't assume the same about its final fields. To work around
 * this, we provide dummy implementations of our collections with their
 * dependencies as non-final fields. Even though these implementations are never
 * instantiated, they are visible to GWT when it performs its serialization
 * analysis, and it assumes that their fields may be serialized.
 *
 * Currently we provide dummy implementations of all the immutable
 * collection classes necessary to support declarations like
 * {@code ImmutableMultiset} in RPC interfaces. Support for
 * {@code ImmutableMultiset} in the interface is support for {@code Multiset},
 * so there is nothing further to be done to support the new collection
 * interfaces. It is not support, however, for an RPC interface in terms of
 * {@code HashMultiset}. It is still possible to send a {@code HashMultiset}
 * over GWT RPC; it is only the declaration of an interface in terms of
 * {@code HashMultiset} that we haven't tried to support. (We may wish to
 * revisit this decision in the future.)
 *
 * @author Chris Povirk
 */
@GwtCompatible
// None of these classes are instantiated, let alone serialized:
@SuppressWarnings("serial")
final class GwtSerializationDependencies {
  private GwtSerializationDependencies() {}

  static final class ImmutableListMultimapDependencies
      extends ImmutableListMultimap {
    K key;
    V value;

    ImmutableListMultimapDependencies() {
      super(null, 0);
    }
  }

  // ImmutableMap is covered by ImmutableSortedMap/ImmutableBiMap.

  // ImmutableMultimap is covered by ImmutableSetMultimap/ImmutableListMultimap.

  static final class ImmutableSetMultimapDependencies
      extends ImmutableSetMultimap {
    K key;
    V value;

    ImmutableSetMultimapDependencies() {
      super(null, 0, null);
    }
  }

  /*
   * We support an interface declared in terms of LinkedListMultimap because it
   * supports entry ordering not supported by other implementations.
   */
  static final class LinkedListMultimapDependencies
      extends LinkedListMultimap {
    K key;
    V value;

    LinkedListMultimapDependencies() {
      super();
    }
  }

  static final class HashBasedTableDependencies
      extends HashBasedTable {
    HashMap> data;

    HashBasedTableDependencies() {
      super(null, null);
    }
  }

  static final class TreeBasedTableDependencies
      extends TreeBasedTable {
    TreeMap> data;

    TreeBasedTableDependencies() {
      super(null, null);
    }
  }

  static final class TreeMultimapDependencies
      extends TreeMultimap {
    Comparator keyComparator;
    Comparator valueComparator;
    K key;
    V value;

    TreeMultimapDependencies() {
      super(null, null);
    }
  }
}