001/*
002 * Copyright 2002-2014 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 */
035public class CompositeIterator<E> implements Iterator<E> {
036
037        private final Set<Iterator<E>> iterators = new LinkedHashSet<Iterator<E>>();
038
039        private boolean inUse = false;
040
041
042        /**
043         * Add given iterator to this composite.
044         */
045        public void add(Iterator<E> iterator) {
046                Assert.state(!this.inUse, "You can no longer add iterators to a composite iterator that's already in use");
047                if (this.iterators.contains(iterator)) {
048                        throw new IllegalArgumentException("You cannot add the same iterator twice");
049                }
050                this.iterators.add(iterator);
051        }
052
053        @Override
054        public boolean hasNext() {
055                this.inUse = true;
056                for (Iterator<E> iterator : this.iterators) {
057                        if (iterator.hasNext()) {
058                                return true;
059                        }
060                }
061                return false;
062        }
063
064        @Override
065        public E next() {
066                this.inUse = true;
067                for (Iterator<E> iterator : this.iterators) {
068                        if (iterator.hasNext()) {
069                                return iterator.next();
070                        }
071                }
072                throw new NoSuchElementException("All iterators exhausted");
073        }
074
075        @Override
076        public void remove() {
077                throw new UnsupportedOperationException("CompositeIterator does not support remove()");
078        }
079
080}