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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.collections4.iterators;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import org.apache.commons.collections4.ResettableIterator;
/**
* An Iterator that restarts when it reaches the end.
*
* The iterator will loop continuously around the provided elements, unless
* there are no elements in the collection to begin with, or all the elements
* have been {@link #remove removed}.
*
* Concurrent modifications are not directly supported, and for most collection
* implementations will throw a ConcurrentModificationException.
*
* @since 3.0
*/
public class LoopingIterator implements ResettableIterator {
/** The collection to base the iterator on */
private final Collection collection;
/** The current iterator */
private Iterator iterator;
/**
* Constructor that wraps a collection.
*
* There is no way to reset an Iterator instance without recreating it from
* the original source, so the Collection must be passed in.
*
* @param coll the collection to wrap
* @throws NullPointerException if the collection is null
*/
public LoopingIterator(final Collection coll) {
if (coll == null) {
throw new NullPointerException("The collection must not be null");
}
collection = coll;
reset();
}
/**
* Has the iterator any more elements.
*
* Returns false only if the collection originally had zero elements, or
* all the elements have been {@link #remove removed}.
*
* @return true if there are more elements
*/
@Override
public boolean hasNext() {
return collection.size() > 0;
}
/**
* Returns the next object in the collection.
*
* If at the end of the collection, return the first element.
*
* @return the next object
* @throws NoSuchElementException if there are no elements
* at all. Use {@link #hasNext} to avoid this error.
*/
@Override
public E next() {
if (collection.size() == 0) {
throw new NoSuchElementException("There are no elements for this iterator to loop on");
}
if (iterator.hasNext() == false) {
reset();
}
return iterator.next();
}
/**
* Removes the previously retrieved item from the underlying collection.
*
* This feature is only supported if the underlying collection's
* {@link Collection#iterator iterator} method returns an implementation
* that supports it.
*
* This method can only be called after at least one {@link #next} method call.
* After a removal, the remove method may not be called again until another
* next has been performed. If the {@link #reset} is called, then remove may
* not be called until {@link #next} is called again.
*/
@Override
public void remove() {
iterator.remove();
}
/**
* Resets the iterator back to the start of the collection.
*/
@Override
public void reset() {
iterator = collection.iterator();
}
/**
* Gets the size of the collection underlying the iterator.
*
* @return the current collection size
*/
public int size() {
return collection.size();
}
}