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 017package com.google.common.collect; 018 019import com.google.common.annotations.GwtCompatible; 020import java.util.NoSuchElementException; 021import javax.annotation.Nullable; 022 023/** 024 * This class provides a skeletal implementation of the {@code Iterator} 025 * interface for sequences whose next element can always be derived from the 026 * previous element. Null elements are not supported, nor is the 027 * {@link #remove()} method. 028 * 029 * <p>Example: <pre> {@code 030 * 031 * Iterator<Integer> powersOfTwo = 032 * new AbstractSequentialIterator<Integer>(1) { 033 * protected Integer computeNext(Integer previous) { 034 * return (previous == 1 << 30) ? null : previous * 2; 035 * } 036 * };}</pre> 037 * 038 * @author Chris Povirk 039 * @since 12.0 (in Guava as {@code AbstractLinkedIterator} since 8.0) 040 */ 041@GwtCompatible 042public abstract class AbstractSequentialIterator<T> extends UnmodifiableIterator<T> { 043 private T nextOrNull; 044 045 /** 046 * Creates a new iterator with the given first element, or, if {@code 047 * firstOrNull} is null, creates a new empty iterator. 048 */ 049 protected AbstractSequentialIterator(@Nullable T firstOrNull) { 050 this.nextOrNull = firstOrNull; 051 } 052 053 /** 054 * Returns the element that follows {@code previous}, or returns {@code null} 055 * if no elements remain. This method is invoked during each call to 056 * {@link #next()} in order to compute the result of a <i>future</i> call to 057 * {@code next()}. 058 */ 059 protected abstract T computeNext(T previous); 060 061 @Override 062 public final boolean hasNext() { 063 return nextOrNull != null; 064 } 065 066 @Override 067 public final T next() { 068 if (!hasNext()) { 069 throw new NoSuchElementException(); 070 } 071 try { 072 return nextOrNull; 073 } finally { 074 nextOrNull = computeNext(nextOrNull); 075 } 076 } 077}