SquirrelJME/SquirrelJME

// -*- Mode: Java; indent-tabs-mode: t; tab-width: 4 -*-
// ---------------------------------------------------------------------------
// SquirrelJME
//     Copyright (C) Stephanie Gawroriski <xer@multiphasicapps.net>
// ---------------------------------------------------------------------------
// SquirrelJME is under the Mozilla Public License Version 2.0.
// See license.mkd for licensing and copyright information.
// ---------------------------------------------------------------------------

package net.multiphasicapps.collections;

import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * This is an iterator which is empty.
 *
 * @since 2016/04/10
 */
public final class EmptyIterator
    implements Iterator
{
    /** The empty iterator. */
    private static Reference<Iterator> _EMPTY_ITERATOR;
    
    /**
     * Initializes the empty iterator.
     *
     * @since 2016/04/10
     */
    private EmptyIterator()
    {
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/04/10
     */
    @Override
    public boolean hasNext()
    {
        return false;
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/04/10
     */
    @Override
    public Object next()
    {
        /* {@squirreljme.error AC02 The empty iterator contains no elements.} */
        throw new NoSuchElementException("AC02");
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/04/10
     */
    @Override
    public void remove()
    {
        /* {@squirreljme.error AC03 Cannot remove elements from the empty
        iterator.} */
        throw new UnsupportedOperationException("AC03");
    }
    
    /**
     * This returns an iterator which contains nothing.
     *
     * @param <V> The type of values to iterate over.
     * @return The empty iterator.
     * @since 2016/04/10
     */
    @SuppressWarnings({"unchecked"})
    public static <V> Iterator<V> empty()
    {
        // Get reference
        Reference<Iterator> ref = EmptyIterator._EMPTY_ITERATOR;
        Iterator rv;
        
        // Needs creation?
        if (ref == null || null == (rv = ref.get()))
            EmptyIterator._EMPTY_ITERATOR = new WeakReference<>(
                (rv = new EmptyIterator()));
        
        // Return it
        return (Iterator<V>)rv;
    }
}