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Neo4j kernel is a lightweight, embedded Java database designed to store data structured as graphs rather than tables. For more information, see http://neo4j.org.

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/*
 * Copyright (c) 2002-2015 "Neo Technology,"
 * Network Engine for Objects in Lund AB [http://neotechnology.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
package org.neo4j.helpers.collection;

import java.util.Collections;
import java.util.Iterator;

/**
 * Combining one or more {@link Iterator}s, making them look like they were
 * one big iterator. All iteration/combining is done lazily.
 *
 * @param  the type of items in the iteration.
 */
public class CombiningIterator extends PrefetchingIterator
{
    private Iterator> iterators;
    private Iterator currentIterator;

    public CombiningIterator( Iterable> iterators )
    {
        this( iterators.iterator() );
    }
    
   public CombiningIterator( Iterator> iterators )
   {
        this.iterators = iterators;
   }

    public CombiningIterator( T first, Iterator rest )
    {
        this( Collections.>emptyList() );
        this.hasFetchedNext = true;
        this.nextObject = first;
        this.currentIterator = rest;
    }

    @Override
    protected T fetchNextOrNull()
    {
        if ( currentIterator == null || !currentIterator.hasNext() )
        {
            while ( (currentIterator = nextIteratorOrNull()) != null )
            {
                if ( currentIterator.hasNext() )
                {
                    break;
                }
            }
        }
        return currentIterator != null && currentIterator.hasNext() ? currentIterator.next() : null;
    }

    protected Iterator nextIteratorOrNull()
    {
        if(iterators.hasNext())
        {
            return iterators.next();
        }
        return null;
    }

    protected Iterator currentIterator()
    {
        return currentIterator;
    }
}