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 * Copyright (C) 2010, Google Inc.
 * and other copyright owners as documented in the project's IP log.
 *
 * This program and the accompanying materials are made available
 * under the terms of the Eclipse Distribution License v1.0 which
 * accompanies this distribution, is reproduced below, and is
 * available at http://www.eclipse.org/org/documents/edl-v10.php
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package org.eclipse.jgit.lib;

import java.io.IOException;

import org.eclipse.jgit.errors.MissingObjectException;

Queue to open objects asynchronously.
A queue may perform background decompression of objects and supply them
(possibly out-of-order) to the application.
Type parameters:
<T> – 
           type of identifier supplied to the call that made the queue./**
 * Queue to open objects asynchronously.
 *
 * A queue may perform background decompression of objects and supply them
 * (possibly out-of-order) to the application.
 *
 * @param <T>
 *            type of identifier supplied to the call that made the queue.
 */
public interface AsyncObjectLoaderQueue<T extends ObjectId> extends
		AsyncOperation {

	
Position this queue onto the next available result. Even if this method returns true, open() may still throw MissingObjectException if the underlying object database was concurrently modified and the current object is no longer available. 
Throws:
MissingObjectException –  the object does not exist. If the implementation is retaining the application's objects getCurrent() will be the current object that is missing. There may be more results still available, so the caller should continue invoking next to examine another result.
IOException – 
            the object store cannot be accessed.
Returns:
true if there is a result available; false if the queue has
        finished its input iteration./**
	 * Position this queue onto the next available result.
	 *
	 * Even if this method returns true, {@link #open()} may still throw
	 * {@link org.eclipse.jgit.errors.MissingObjectException} if the underlying
	 * object database was concurrently modified and the current object is no
	 * longer available.
	 *
	 * @return true if there is a result available; false if the queue has
	 *         finished its input iteration.
	 * @throws org.eclipse.jgit.errors.MissingObjectException
	 *             the object does not exist. If the implementation is retaining
	 *             the application's objects {@link #getCurrent()} will be the
	 *             current object that is missing. There may be more results
	 *             still available, so the caller should continue invoking next
	 *             to examine another result.
	 * @throws java.io.IOException
	 *             the object store cannot be accessed.
	 */
	boolean next() throws MissingObjectException, IOException;

	
Get the current object, null if the implementation lost track.
Returns:
the current object, null if the implementation lost track. Implementations may for performance reasons discard the caller's ObjectId and provider their own through getObjectId()./**
	 * Get the current object, null if the implementation lost track.
	 *
	 * @return the current object, null if the implementation lost track.
	 *         Implementations may for performance reasons discard the caller's
	 *         ObjectId and provider their own through {@link #getObjectId()}.
	 */
	T getCurrent();

	
Get the ObjectId of the current object. Never null.
Returns:
the ObjectId of the current object. Never null./**
	 * Get the ObjectId of the current object. Never null.
	 *
	 * @return the ObjectId of the current object. Never null.
	 */
	ObjectId getObjectId();

	
Obtain a loader to read the object.
This method can only be invoked once per result
Due to race conditions with a concurrent modification of the underlying
object database, an object may be unavailable when this method is
invoked, even though next returned successfully.
Throws:
MissingObjectException –  the object does not exist. If the implementation is retaining the application's objects getCurrent() will be the current object that is missing. There may be more results still available, so the caller should continue invoking next to examine another result.
IOException – 
            the object store cannot be accessed.
Returns:
the ObjectLoader to read this object. Never null./**
	 * Obtain a loader to read the object.
	 *
	 * This method can only be invoked once per result
	 *
	 * Due to race conditions with a concurrent modification of the underlying
	 * object database, an object may be unavailable when this method is
	 * invoked, even though next returned successfully.
	 *
	 * @return the ObjectLoader to read this object. Never null.
	 * @throws MissingObjectException
	 *             the object does not exist. If the implementation is retaining
	 *             the application's objects {@link #getCurrent()} will be the
	 *             current object that is missing. There may be more results
	 *             still available, so the caller should continue invoking next
	 *             to examine another result.
	 * @throws java.io.IOException
	 *             the object store cannot be accessed.
	 */
	ObjectLoader open() throws IOException;
}