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;
  
  /**
   * A DocIdSet contains a set of doc ids. Implementing classes must
   * only implement {@link #iterator} to provide access to the set. 
   */
  public abstract class DocIdSet {
  
    /** An empty {@code DocIdSet} instance for easy use, e.g. in Filters that hit no documents. */
    public static final DocIdSet EMPTY_DOCIDSET = new DocIdSet() {
      
      private final DocIdSetIterator iterator = new DocIdSetIterator() {
        @Override
        public int advance(int target) throws IOException { return NO_MORE_DOCS; }
        @Override
        public int docID() { return NO_MORE_DOCS; }
        @Override
        public int nextDoc() throws IOException { return NO_MORE_DOCS; }
      };
      
      @Override
      public DocIdSetIterator iterator() {
        return iterator;
      }
      
      @Override
      public boolean isCacheable() {
        return true;
      }
    };
      
    /** Provides a {@link DocIdSetIterator} to access the set.
     * This implementation can return <code>null</code> or
     * <code>{@linkplain #EMPTY_DOCIDSET}.iterator()</code> if there
     * are no docs that match. */
    public abstract DocIdSetIterator iterator() throws IOException;
  
    /**
     * This method is a hint for {@link CachingWrapperFilter}, if this <code>DocIdSet</code>
     * should be cached without copying it into a BitSet. The default is to return
     * <code>false</code>. If you have an own <code>DocIdSet</code> implementation
     * that does its iteration very effective and fast without doing disk I/O,
     * override this method and return <code>true</here>.
     */
    public boolean isCacheable() {
      return false;
    }
  }
  