1   package org.apache.lucene.search;
    2   
    3   /**
    4    * Licensed to the Apache Software Foundation (ASF) under one or more
    5    * contributor license agreements.  See the NOTICE file distributed with
    6    * this work for additional information regarding copyright ownership.
    7    * The ASF licenses this file to You under the Apache License, Version 2.0
    8    * (the "License"); you may not use this file except in compliance with
    9    * the License.  You may obtain a copy of the License at
   10    *
   11    *     http://www.apache.org/licenses/LICENSE-2.0
   12    *
   13    * Unless required by applicable law or agreed to in writing, software
   14    * distributed under the License is distributed on an "AS IS" BASIS,
   15    * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   16    * See the License for the specific language governing permissions and
   17    * limitations under the License.
   18    */
   19   
   20   import java.io.IOException;
   21   
   22   import org.apache.lucene.index.IndexReader;
   23   import org.apache.lucene.util.DocIdBitSet;
   24   
   25   /** 
   26    *  Abstract base class for restricting which documents may
   27    *  be returned during searching.
   28    */
   29   public abstract class Filter implements java.io.Serializable {
   30     
   31     /**
   32      * Creates a {@link DocIdSet} enumerating the documents that should be
   33      * permitted in search results. <b>NOTE:</b> null can be
   34      * returned if no documents are accepted by this Filter.
   35      * <p>
   36      * Note: This method will be called once per segment in
   37      * the index during searching.  The returned {@link DocIdSet}
   38      * must refer to document IDs for that segment, not for
   39      * the top-level reader.
   40      * 
   41      * @param reader a {@link IndexReader} instance opened on the index currently
   42      *         searched on. Note, it is likely that the provided reader does not
   43      *         represent the whole underlying index i.e. if the index has more than
   44      *         one segment the given reader only represents a single segment.
   45      *          
   46      * @return a DocIdSet that provides the documents which should be permitted or
   47      *         prohibited in search results. <b>NOTE:</b> null can be returned if
   48      *         no documents will be accepted by this Filter.
   49      * 
   50      * @see DocIdBitSet
   51      */
   52     public abstract DocIdSet getDocIdSet(IndexReader reader) throws IOException;
   53   }