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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     * <p>Example: <pre>   {@code
033     *
034     *   Iterator<Integer> powersOfTwo = new AbstractLinkedIterator<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 8.0
042     */
043    @Beta
044    @GwtCompatible
045    public abstract class AbstractLinkedIterator<T>
046        extends UnmodifiableIterator<T> {
047      private T nextOrNull;
048    
049      /**
050       * Creates a new iterator with the given first element, or, if {@code
051       * firstOrNull} is null, creates a new empty iterator.
052       */
053      protected AbstractLinkedIterator(@Nullable T firstOrNull) {
054        this.nextOrNull = firstOrNull;
055      }
056    
057      /**
058       * Returns the element that follows {@code previous}, or returns {@code null}
059       * if no elements remain. This method is invoked during each call to
060       * {@link #next()} in order to compute the result of a <i>future</i> call to
061       * {@code next()}.
062       */
063      protected abstract T computeNext(T previous);
064    
065      @Override
066      public final boolean hasNext() {
067        return nextOrNull != null;
068      }
069    
070      @Override
071      public final T next() {
072        if (!hasNext()) {
073          throw new NoSuchElementException();
074        }
075        try {
076          return nextOrNull;
077        } finally {
078          nextOrNull = computeNext(nextOrNull);
079        }
080      }
081    }