001/* 002 * Copyright 2002-2018 the original author or 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 * https://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 org.springframework.util; 018 019import java.util.Iterator; 020import java.util.LinkedHashSet; 021import java.util.NoSuchElementException; 022import java.util.Set; 023 024/** 025 * Composite iterator that combines multiple other iterators, 026 * as registered via {@link #add(Iterator)}. 027 * 028 * <p>This implementation maintains a linked set of iterators 029 * which are invoked in sequence until all iterators are exhausted. 030 * 031 * @author Erwin Vervaet 032 * @author Juergen Hoeller 033 * @since 3.0 034 * @param <E> the element type 035 */ 036public class CompositeIterator<E> implements Iterator<E> { 037 038 private final Set<Iterator<E>> iterators = new LinkedHashSet<>(); 039 040 private boolean inUse = false; 041 042 043 /** 044 * Add given iterator to this composite. 045 */ 046 public void add(Iterator<E> iterator) { 047 Assert.state(!this.inUse, "You can no longer add iterators to a composite iterator that's already in use"); 048 if (this.iterators.contains(iterator)) { 049 throw new IllegalArgumentException("You cannot add the same iterator twice"); 050 } 051 this.iterators.add(iterator); 052 } 053 054 @Override 055 public boolean hasNext() { 056 this.inUse = true; 057 for (Iterator<E> iterator : this.iterators) { 058 if (iterator.hasNext()) { 059 return true; 060 } 061 } 062 return false; 063 } 064 065 @Override 066 public E next() { 067 this.inUse = true; 068 for (Iterator<E> iterator : this.iterators) { 069 if (iterator.hasNext()) { 070 return iterator.next(); 071 } 072 } 073 throw new NoSuchElementException("All iterators exhausted"); 074 } 075 076 @Override 077 public void remove() { 078 throw new UnsupportedOperationException("CompositeIterator does not support remove()"); 079 } 080 081}