    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
    017    package com.google.common.collect;
    018
    019    import com.google.common.annotations.GwtCompatible;
    020
    021    import java.util.NoSuchElementException;
    022
    023    import javax.annotation.Nullable;
    024
    025    /**
    026     * This class provides a skeletal implementation of the {@code Iterator}
    027     * interface for sequences whose next element can always be derived from the
    028     * previous element. Null elements are not supported, nor is the
    029     * {@link #remove()} method.
    030     *
    031     * <p>Example: <pre>   {@code
    032     *
    033     *   Iterator<Integer> powersOfTwo =
    034     *       new AbstractSequentialIterator<Integer>(1) {
    035     *         protected Integer computeNext(Integer previous) {
    036     *           return (previous == 1 << 30) ? null : previous * 2;
    037     *         }
    038     *       };}</pre>
    039     *
    040     * @author Chris Povirk
    041     * @since 12.0 (in Guava as {@code AbstractLinkedIterator} since 8.0)
    042     */
    043    @GwtCompatible
    044    public abstract class AbstractSequentialIterator<T>
    045        extends UnmodifiableIterator<T> {
    046      private T nextOrNull;
    047
    048      /**
    049       * Creates a new iterator with the given first element, or, if {@code
    050       * firstOrNull} is null, creates a new empty iterator.
    051       */
    052      protected AbstractSequentialIterator(@Nullable T firstOrNull) {
    053        this.nextOrNull = firstOrNull;
    054      }
    055
    056      /**
    057       * Returns the element that follows {@code previous}, or returns {@code null}
    058       * if no elements remain. This method is invoked during each call to
    059       * {@link #next()} in order to compute the result of a <i>future</i> call to
    060       * {@code next()}.
    061       */
    062      protected abstract T computeNext(T previous);
    063
    064      @Override
    065      public final boolean hasNext() {
    066        return nextOrNull != null;
    067      }
    068
    069      @Override
    070      public final T next() {
    071        if (!hasNext()) {
    072          throw new NoSuchElementException();
    073        }
    074        try {
    075          return nextOrNull;
    076        } finally {
    077          nextOrNull = computeNext(nextOrNull);
    078        }
    079      }
    080    }