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Relational Persistence for Java

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//$Id: JoinedIterator.java 3973 2004-06-28 23:58:08Z epbernard $
package org.hibernate.util;

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

/**
 * An JoinedIterator is an Iterator that wraps a number of Iterators.
 *
 * This class makes multiple iterators look like one to the caller.
 * When any method from the Iterator interface is called, the JoinedIterator
 * will delegate to a single underlying Iterator. The JoinedIterator will
 * invoke the Iterators in sequence until all Iterators are exhausted.
 *
 */
public class JoinedIterator implements Iterator {

	private static final Iterator[] ITERATORS = {};

	// wrapped iterators
	private Iterator[] iterators;

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

	// the current iterator
	private Iterator currentIterator;

	// the last used iterator
	private Iterator lastUsedIterator;

	public JoinedIterator(List iterators) {
		this( (Iterator[]) iterators.toArray(ITERATORS) );
	}

	public JoinedIterator(Iterator[] iterators) {
		if( iterators==null )
			throw new NullPointerException("Unexpected NULL iterators argument");
		this.iterators = iterators;
	}

	public JoinedIterator(Iterator first, Iterator second) {
		this( new Iterator[] { first, second } );
	}

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

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

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


	// call this before any Iterator method to make sure that the current Iterator
	// is not exhausted
	protected void updateCurrentIterator() {

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

		while (! currentIterator.hasNext() && currentIteratorIndex < iterators.length - 1) {
			currentIteratorIndex++;
			currentIterator = iterators[currentIteratorIndex];
		}
	}

}