    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.Beta;
    020    import com.google.common.annotations.GwtCompatible;
    021
    022    import java.util.NoSuchElementException;
    023
    024    import 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 8.0
    034     */
    035    @Beta
    036    @GwtCompatible
    037    public 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    }