content: package org.apache.lucene.search;
  
  /**
   * 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.IOException;
  
  import org.apache.lucene.index.IndexReader;
  import org.apache.lucene.util.DocIdBitSet;
  
  /** 
   *  Abstract base class for restricting which documents may
   *  be returned during searching.
   */
  public abstract class Filter implements java.io.Serializable {
    
    /**
     * Creates a {@link DocIdSet} enumerating the documents that should be
     * permitted in search results. <b>NOTE:</b> null can be
     * returned if no documents are accepted by this Filter.
     * <p>
     * Note: This method will be called once per segment in
     * the index during searching.  The returned {@link DocIdSet}
     * must refer to document IDs for that segment, not for
     * the top-level reader.
     * 
     * @param reader a {@link IndexReader} instance opened on the index currently
     *         searched on. Note, it is likely that the provided reader does not
     *         represent the whole underlying index i.e. if the index has more than
     *         one segment the given reader only represents a single segment.
     *          
     * @return a DocIdSet that provides the documents which should be permitted or
     *         prohibited in search results. <b>NOTE:</b> null can be returned if
     *         no documents will be accepted by this Filter.
     * 
     * @see DocIdBitSet
     */
    public abstract DocIdSet getDocIdSet(IndexReader reader) throws IOException;
  }
  