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 }