Java tutorial
/* * 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. */ package org.apache.lucene.search; import java.io.IOException; import org.apache.lucene.util.Accountable; import org.apache.lucene.util.Bits; /** * 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 implements Accountable { /** An empty {@code DocIdSet} instance */ public static final DocIdSet EMPTY = new DocIdSet() { @Override public DocIdSetIterator iterator() { return DocIdSetIterator.empty(); } // we explicitly provide no random access, as this filter is 100% sparse and iterator exits faster @Override public Bits bits() { return null; } @Override public long ramBytesUsed() { return 0L; } }; /** Provides a {@link DocIdSetIterator} to access the set. * This implementation can return <code>null</code> if there * are no docs that match. */ public abstract DocIdSetIterator iterator() throws IOException; // TODO: somehow this class should express the cost of // iteration vs the cost of random access Bits; for // expensive Filters (e.g. distance < 1 km) we should use // bits() after all other Query/Filters have matched, but // this is the opposite of what bits() is for now // (down-low filtering using e.g. FixedBitSet) /** Optionally provides a {@link Bits} interface for random access * to matching documents. * @return {@code null}, if this {@code DocIdSet} does not support random access. * In contrast to {@link #iterator()}, a return value of {@code null} * <b>does not</b> imply that no documents match the filter! * The default implementation does not provide random access, so you * only need to implement this method if your DocIdSet can * guarantee random access to every docid in O(1) time without * external disk access (as {@link Bits} interface cannot throw * {@link IOException}). This is generally true for bit sets * like {@link org.apache.lucene.util.FixedBitSet}, which return * itself if they are used as {@code DocIdSet}. */ public Bits bits() throws IOException { return null; } }