content: package org.apache.lucene.util;
  
  /**
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You 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.
   */
  
  import java.io.Serializable;
  import java.util.AbstractSet;
  import java.util.Iterator;
  import java.util.Map;
  
  /**
   * A Set implementation that wraps an actual Map based
   * implementation.
   * 
   * @lucene.internal
   */
  public final class MapBackedSet<E> extends AbstractSet<E> implements Serializable {
  
    private static final long serialVersionUID = -6761513279741915432L;
  
    private final Map<E, Boolean> map;
  
    /**
     * Creates a new instance which wraps the specified {@code map}.
     */
    public MapBackedSet(Map<E, Boolean> map) {
      this.map = map;
    }
  
    @Override
    public int size() {
      return map.size();
    }
  
    @Override
    public boolean contains(Object o) {
      return map.containsKey(o);
    }
  
    @Override
    public boolean add(E o) {
      return map.put(o, Boolean.TRUE) == null;
    }
  
    @Override
    public boolean remove(Object o) {
      return map.remove(o) != null;
    }
  
    @Override
    public void clear() {
      map.clear();
    }
  
    @Override
    public Iterator<E> iterator() {
      return map.keySet().iterator();
    }
  }
  