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package org.apache.commons.collections.iterators;

import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;

import org.apache.commons.collections.ResettableIterator;

/**
 * An Iterator that restarts when it reaches the end.
 * <p>
 * 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}.
 * <p>
 * Concurrent modifications are not directly supported, and for most collection
 * implementations will throw a ConcurrentModificationException. 
 *
 * @since Commons Collections 3.0
 * @version $Revision: 3330 $ $Date: 2006-03-14 09:47:58 +0100 (Di, 14 Mrz 2006) $
 *
 * @author <a href="mailto:joncrlsn@users.sf.net">Jonathan Carlson</a>
 * @author Stephen Colebourne
 */
public class LoopingIterator implements ResettableIterator {
    
    /** The collection to base the iterator on */
    private Collection collection;
    /** The current iterator */
    private Iterator iterator;

    /**
     * Constructor that wraps a collection.
     * <p>
     * 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(Collection coll) {
        if (coll == null) {
            throw new NullPointerException("The collection must not be null");
        }
        collection = coll;
        reset();
    }

    /** 
     * Has the iterator any more elements.
     * <p>
     * Returns false only if the collection originally had zero elements, or
     * all the elements have been {@link #remove removed}.
     * 
     * @return <code>true</code> if there are more elements
     */
    public boolean hasNext() {
        return (collection.size() > 0);
    }

    /**
     * Returns the next object in the collection.
     * <p>
     * If at the end of the collection, return the first element.
     * 
     * @throws NoSuchElementException if there are no elements
     *         at all.  Use {@link #hasNext} to avoid this error.
     */
    public Object 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.
     * <p>
     * This feature is only supported if the underlying collection's 
     * {@link Collection#iterator iterator} method returns an implementation 
     * that supports it.
     * <p>
     * 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.
     */
    public void remove() {
        iterator.remove();
    }

    /**
     * Resets the iterator back to the start of the collection.
     */
    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();
    }

}
