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The AWS SDK for Java with support for OSGi. The AWS SDK for Java provides Java APIs for building software on AWS' cost-effective, scalable, and reliable infrastructure products. The AWS Java SDK allows developers to code against APIs for all of Amazon's infrastructure web services (Amazon S3, Amazon EC2, Amazon SQS, Amazon Relational Database Service, Amazon AutoScaling, etc).

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/*
 * Copyright 2014-2016 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License").
 * You may not use this file except in compliance with the License.
 * A copy of the License is located at
 *
 *  http://aws.amazon.com/apache2.0
 *
 * or in the "license" file accompanying this file. This file is distributed
 * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
 * express or implied. See the License for the specific language governing
 * permissions and limitations under the License.
 */
package com.amazonaws.services.dynamodbv2.document.internal;

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

import com.amazonaws.services.dynamodbv2.document.Page;

/**
 * An internal iterator implementation for {@link PageBasedCollection}.
 * 
 * NOTE: this internal class is marked as public since it has been incorrectly
 * exposed in the public method {@link PageBasedCollection#iterator()}, and it
 * will be changed to be package private in the next major version.
 *
 * @param 
 *            resource type
 * @param 
 *            low level result type
 */
public class IteratorSupport implements Iterator {
    /**
     * Used to iterate through the resource pages, dynamically making network
     * calls as needed.
     */
    final PageIterator resourcePageIterator;
    /**
     * Used to iterate through a list of resources already retrieved.
     */
    private Iterator localResourceIterator;
    private T resource;

    IteratorSupport(PageIterator resourcePageIterator) {
        this.resourcePageIterator = resourcePageIterator;
    }

    @Override
    public boolean hasNext() {
        if (resource != null) {
            return true;
        }
        resource = nextResource();
        return (resource != null);
    }

    @Override
    public T next() {
        T rval = resource;
        if (rval == null) {
            rval = nextResource();
            if (rval == null) {
                throw new NoSuchElementException("No more elements");
            }
        } else {
            resource = null;
        }
        return rval;
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException(
                "Collection is read-only");
    }

    private T nextResource() {
        while (true) {
            if (localResourceIterator != null && localResourceIterator.hasNext()) {
                return localResourceIterator.next();
            }
            if (!resourcePageIterator.hasNext()) {
                return null;
            }
            Page resourcePage = resourcePageIterator.next();
            localResourceIterator = resourcePage.iterator();
        }
    }
}