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 = new AbstractSequentialIterator<Integer>(1) {
034     *     protected Integer computeNext(Integer previous) {
035     *       return (previous == 1 << 30) ? null : previous * 2;
036     *     }
037     *   };}</pre>
038     *
039     * @author Chris Povirk
040     * @since 12.0 (in Guava as {@code AbstractLinkedIterator} since 8.0)
041     */
042    @GwtCompatible
043    public abstract class AbstractSequentialIterator<T>
044        extends UnmodifiableIterator<T> {
045      private T nextOrNull;
046    
047      /**
048       * Creates a new iterator with the given first element, or, if {@code
049       * firstOrNull} is null, creates a new empty iterator.
050       */
051      protected AbstractSequentialIterator(@Nullable T firstOrNull) {
052        this.nextOrNull = firstOrNull;
053      }
054    
055      /**
056       * Returns the element that follows {@code previous}, or returns {@code null}
057       * if no elements remain. This method is invoked during each call to
058       * {@link #next()} in order to compute the result of a <i>future</i> call to
059       * {@code next()}.
060       */
061      protected abstract T computeNext(T previous);
062    
063      @Override
064      public final boolean hasNext() {
065        return nextOrNull != null;
066      }
067    
068      @Override
069      public final T next() {
070        if (!hasNext()) {
071          throw new NoSuchElementException();
072        }
073        try {
074          return nextOrNull;
075        } finally {
076          nextOrNull = computeNext(nextOrNull);
077        }
078      }
079    }