/*
 * 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.index;

import java.io.IOException;
import java.util.Iterator;
import org.apache.lucene.codecs.FieldsProducer;

/**
 * Provides a {@link Terms} index for fields that have it, and lists which fields do. This is
 * primarily an internal/experimental API (see {@link FieldsProducer}), although it is also used to
 * expose the set of term vectors per document.
 *
 * @lucene.experimental
 */
public abstract class Fields implements Iterable<String> {

  /** Sole constructor. (For invocation by subclass constructors, typically implicit.) */
  protected Fields() {}

  /** Returns an iterator that will step through all fields names. This will not return null. */
  @Override
  public abstract Iterator<String> iterator();

  /** Get the {@link Terms} for this field. This will return null if the field does not exist. */
  public abstract Terms terms(String field) throws IOException;

  /**
   * Returns the number of fields or -1 if the number of distinct field names is unknown. If &gt;=
   * 0, {@link #iterator} will return as many field names.
   */
  public abstract int size();
}
