lucene/core/src/java/org/apache/lucene/search/Query.java

/*
 * 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.index.IndexReader;

/** The abstract base class for queries.
    <p>Instantiable subclasses are:
    <ul>
    <li> {@link TermQuery}
    <li> {@link BooleanQuery}
    <li> {@link WildcardQuery}
    <li> {@link PhraseQuery}
    <li> {@link PrefixQuery}
    <li> {@link MultiPhraseQuery}
    <li> {@link FuzzyQuery}
    <li> {@link RegexpQuery}
    <li> {@link TermRangeQuery}
    <li> {@link PointRangeQuery}
    <li> {@link ConstantScoreQuery}
    <li> {@link DisjunctionMaxQuery}
    <li> {@link MatchAllDocsQuery}
    </ul>
    <p>See also the family of {@link org.apache.lucene.search.spans Span Queries}
       and additional queries available in the <a href="{@docRoot}/../queries/overview-summary.html">Queries module</a>
*/
public abstract class Query {

  /** Prints a query to a string, with <code>field</code> assumed to be the 
   * default field and omitted.
   */
  public abstract String toString(String field);

  /** Prints a query to a string. */
  @Override
  public final String toString() {
    return toString("");
  }

  /**
   * Expert: Constructs an appropriate Weight implementation for this query.
   * <p>
   * Only implemented by primitive queries, which re-write to themselves.
   *
   * @param scoreMode     How the produced scorers will be consumed.
   * @param boost         The boost that is propagated by the parent queries.
   */
  public Weight createWeight(IndexSearcher searcher, ScoreMode scoreMode, float boost) throws IOException {
    throw new UnsupportedOperationException("Query " + this + " does not implement createWeight");
  }

  /** Expert: called to re-write queries into primitive queries. For example,
   * a PrefixQuery will be rewritten into a BooleanQuery that consists
   * of TermQuerys.
   */
  public Query rewrite(IndexReader reader) throws IOException {
    return this;
  }

  /**
   * Recurse through the query tree, visiting any child queries
   * @param visitor a QueryVisitor to be called by each query in the tree
   */
  public void visit(QueryVisitor visitor) {
    visitor.visitLeaf(this);
  }

  /**
   * Override and implement query instance equivalence properly in a subclass. 
   * This is required so that {@link QueryCache} works properly.
   * 
   * Typically a query will be equal to another only if it's an instance of 
   * the same class and its document-filtering properties are identical that other
   * instance. Utility methods are provided for certain repetitive code. 
   * 
   * @see #sameClassAs(Object)
   * @see #classHash()
   */
  @Override
  public abstract boolean equals(Object obj);

  /**
   * Override and implement query hash code properly in a subclass. 
   * This is required so that {@link QueryCache} works properly.
   * 
   * @see #equals(Object)
   */
  @Override
  public abstract int hashCode();

  /**
   * Utility method to check whether <code>other</code> is not null and is exactly 
   * of the same class as this object's class.
   * 
   * When this method is used in an implementation of {@link #equals(Object)},
   * consider using {@link #classHash()} in the implementation
   * of {@link #hashCode} to differentiate different class
   */
  protected final boolean sameClassAs(Object other) {
    return other != null && getClass() == other.getClass();
  }

  private final int CLASS_NAME_HASH = getClass().getName().hashCode();

  /**
   * Provides a constant integer for a given class, derived from the name of the class.
   * The rationale for not using just {@link Class#hashCode()} is that classes may be
   * assigned different hash codes for each execution and we want hashes to be possibly
   * consistent to facilitate debugging.    
   */
  protected final int classHash() {
    return CLASS_NAME_HASH;
  }
}