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    }