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