001/*
002 * Copyright (C) 2010 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import com.google.common.annotations.Beta;
020import com.google.common.annotations.GwtCompatible;
021
022import java.util.NoSuchElementException;
023
024import javax.annotation.Nullable;
025
026/**
027 * This class provides a skeletal implementation of the {@code Iterator}
028 * interface for sequences whose next element can always be derived from the
029 * previous element. Null elements are not supported, nor is the
030 * {@link #remove()} method.
031 *
032 * @author Chris Povirk
033 * @since 8
034 */
035@Beta
036@GwtCompatible
037public abstract class AbstractLinkedIterator<T>
038    extends UnmodifiableIterator<T> {
039  private T nextOrNull;
040
041  /**
042   * Creates a new iterator with the given first element, or, if {@code
043   * firstOrNull} is null, creates a new empty iterator.
044   */
045  protected AbstractLinkedIterator(@Nullable T firstOrNull) {
046    this.nextOrNull = firstOrNull;
047  }
048
049  /**
050   * Returns the element that follows {@code previous}, or returns {@code null}
051   * if no elements remain. This method is invoked during each call to
052   * {@link #next()} in order to compute the result of a <i>future</i> call to
053   * {@code next()}.
054   */
055  protected abstract T computeNext(T previous);
056
057  @Override
058  public final boolean hasNext() {
059    return nextOrNull != null;
060  }
061
062  @Override
063  public final T next() {
064    if (!hasNext()) {
065      throw new NoSuchElementException();
066    }
067    try {
068      return nextOrNull;
069    } finally {
070      nextOrNull = computeNext(nextOrNull);
071    }
072  }
073}