/*
    * Copyright (C) 2010 The Guava Authors
    *
    * Licensed under the Apache License, Version 2.0 (the "License");
    * you may not use this file except in compliance with the License.
    * You may obtain a copy of the License at
    *
    * http://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package com.google.common.collect;
  
  import com.google.common.annotations.GwtCompatible;
  
  import java.util.NoSuchElementException;
  
  import javax.annotation.Nullable;
  
  /**
   * This class provides a skeletal implementation of the {@code Iterator}
   * interface for sequences whose next element can always be derived from the
   * previous element. Null elements are not supported, nor is the
   * {@link #remove()} method.
   *
   * <p>Example: <pre>   {@code
   *
   *   Iterator<Integer> powersOfTwo = 
   *       new AbstractSequentialIterator<Integer>(1) {
   *         protected Integer computeNext(Integer previous) {
   *           return (previous == 1 << 30) ? null : previous * 2;
   *         }
   *       };}</pre>
   *
   * @author Chris Povirk
   * @since 12.0 (in Guava as {@code AbstractLinkedIterator} since 8.0)
   */
  @GwtCompatible
  public abstract class AbstractSequentialIterator<T>
      extends UnmodifiableIterator<T> {
    private T nextOrNull;
  
    /**
     * Creates a new iterator with the given first element, or, if {@code
     * firstOrNull} is null, creates a new empty iterator.
     */
    protected AbstractSequentialIterator(@Nullable T firstOrNull) {
      this.nextOrNull = firstOrNull;
    }
  
    /**
     * Returns the element that follows {@code previous}, or returns {@code null}
     * if no elements remain. This method is invoked during each call to
     * {@link #next()} in order to compute the result of a <i>future</i> call to
     * {@code next()}.
     */
    protected abstract T computeNext(T previous);
  
    @Override
    public final boolean hasNext() {
      return nextOrNull != null;
    }
  
    @Override
    public final T next() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      try {
        return nextOrNull;
      } finally {
        nextOrNull = computeNext(nextOrNull);
      }
    }
  }