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