https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * 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;


Implementation of ImmutableListMultimap with no entries. 
Author:Mike Ward
/**
 * Implementation of {@link ImmutableListMultimap} with no entries.
 *
 * @author Mike Ward
 */
@GwtCompatible(serializable = true)
class EmptyImmutableSetMultimap extends ImmutableSetMultimap<Object, Object> {
  static final EmptyImmutableSetMultimap INSTANCE = new EmptyImmutableSetMultimap();

  private EmptyImmutableSetMultimap() {
    super(ImmutableMap.<Object, ImmutableSet<Object>>of(), 0, null);
  }

  private Object readResolve() {
    return INSTANCE; // preserve singleton property
  }

  private static final long serialVersionUID = 0;
}