org.neo4j.helpers.collection.PrefetchingIterator Maven / Gradle / Ivy

Go to download

Show more of this group Show more artifacts with this name
Show all versions of neo4j-kernel Show documentation

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.

There is a newer version: 5.26.0

Show newest version

/*
 * 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.Iterator;
import java.util.NoSuchElementException;

/**
 * Abstract class for how you usually implement iterators when you don't know
 * how many objects there are (which is pretty much every time)
 *
 * Basically the {@link #hasNext()} method will look up the next object and
 * cache it. The cached object is then set to {@code null} in {@link #next()}.
 * So you only have to implement one method, {@code fetchNextOrNull} which
 * returns {@code null} when the iteration has reached the end, and you're done.
 */
public abstract class PrefetchingIterator implements Iterator
{
    boolean hasFetchedNext;
    T nextObject;

	/**
	 * @return {@code true} if there is a next item to be returned from the next
	 * call to {@link #next()}.
	 */
	@Override
    public boolean hasNext()
	{
		return peek() != null;
	}

    /**
     * @return the next element that will be returned from {@link #next()} without
     * actually advancing the iterator
     */
    public T peek()
    {
        if ( hasFetchedNext )
        {
            return nextObject;
        }

        nextObject = fetchNextOrNull();
        hasFetchedNext = true;
        return nextObject;
    }

    /**
	 * Uses {@link #hasNext()} to try to fetch the next item and returns it
	 * if found, otherwise it throws a {@link NoSuchElementException}.
	 *
	 * @return the next item in the iteration, or throws
	 * {@link NoSuchElementException} if there's no more items to return.
	 */
	@Override
    public T next()
	{
		if ( !hasNext() )
		{
			throw new NoSuchElementException();
		}
		T result = nextObject;
		nextObject = null;
		hasFetchedNext = false;
		return result;
	}

	protected abstract T fetchNextOrNull();

	@Override
    public void remove()
	{
		throw new UnsupportedOperationException();
	}
}