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The core O/RM functionality as provided by Hibernate

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/*
 * Hibernate, Relational Persistence for Idiomatic Java
 *
 * License: GNU Lesser General Public License (LGPL), version 2.1 or later.
 * See the lgpl.txt file in the root directory or .
 */
package org.hibernate.internal.util.collections;

import java.util.Iterator;
import java.util.List;


/**
 * An JoinedIterable is an Iterable that wraps a number of Iterables.
 *
 * This class makes multiple iterables look like one to the caller.
 * When any method from the Iterator interface is called on the
 * Iterator object returned by {@link #iterator()}, the JoinedIterable
 * will delegate to a single underlying Iterator. The JoinedIterable will
 * invoke the iterator on each Iterable, in sequence, until all Iterators
 * are exhausted.
 *
 * @author Gail Badner (adapted from JoinedIterator)
 */
public class JoinedIterable implements Iterable {
	private final TypeSafeJoinedIterator iterator;

	public JoinedIterable(List> iterables) {
		if ( iterables == null ) {
			throw new NullPointerException( "Unexpected null iterables argument" );
		}
		iterator = new TypeSafeJoinedIterator( iterables );
	}

	public Iterator iterator() {
		return iterator;
	}

	private static class TypeSafeJoinedIterator implements Iterator {

		// wrapped iterators
		private List> iterables;

		// index of current iterator in the wrapped iterators array
		private int currentIterableIndex;

		// the current iterator
		private Iterator currentIterator;

		// the last used iterator
		private Iterator lastUsedIterator;

		public TypeSafeJoinedIterator(List> iterables) {
			this.iterables = iterables;
		}

		public boolean hasNext() {
			updateCurrentIterator();
			return currentIterator.hasNext();
		}

		public T next() {
			updateCurrentIterator();
			return currentIterator.next();
		}

		public void remove() {
			updateCurrentIterator();
			lastUsedIterator.remove();
		}

		// call this beforeQuery any Iterator method to make sure that the current Iterator
		// is not exhausted
		@SuppressWarnings( {"unchecked"})
		protected void updateCurrentIterator() {

			if ( currentIterator == null) {
				if( iterables.size() == 0  ) {
					currentIterator = EmptyIterator.INSTANCE;
				}
				else {
					currentIterator = iterables.get( 0 ).iterator();
				}
				// set last used iterator here, in case the user calls remove
				// beforeQuery calling hasNext() or next() (although they shouldn't)
				lastUsedIterator = currentIterator;
			}

			while (! currentIterator.hasNext() && currentIterableIndex < iterables.size() - 1) {
				currentIterableIndex++;
				currentIterator = iterables.get( currentIterableIndex ).iterator();
			}
		}
	}
}