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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. Guava has only one code dependency - javax.annotation, per the JSR-305 spec.

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/*
 * Copyright (C) 2008 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 javax.annotation.Nullable;

/**
 * Bimap with no mappings.
 *
 * @author Jared Levy
 */
@GwtCompatible(emulated = true)
@SuppressWarnings("serial") // uses writeReplace(), not default serialization
final class EmptyImmutableBiMap extends ImmutableBiMap {
  static final EmptyImmutableBiMap INSTANCE = new EmptyImmutableBiMap();

  private EmptyImmutableBiMap() {}
  
  @Override public ImmutableBiMap inverse() {
    return this;
  }
  
  @Override
  public int size() {
    return 0;
  }

  @Override
  public boolean isEmpty() {
    return true;
  }

  @Override
  public Object get(@Nullable Object key) {
    return null;
  }

  @Override
  public ImmutableSet> entrySet() {
    return ImmutableSet.of();
  }

  @Override
  ImmutableSet> createEntrySet() {
    throw new AssertionError("should never be called");
  }

  @Override
  public ImmutableSetMultimap asMultimap() {
    return ImmutableSetMultimap.of();
  }

  @Override
  public ImmutableSet