org.grouplens.common.cursors.Cursors Maven / Gradle / Ivy

/*
 * GroupLens Common Utilities
 * Copyright © 2011 Regents of the University of Minnesota
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *  * Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 *  * Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the
 *    distribution.
 *
 *  * Neither the name of the University of Minnesota nor the names of
 *    its contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
package org.grouplens.common.cursors;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;

import com.google.common.base.Predicate;

/**
 * Utility methods for cursors.
 * @author Michael Ekstrand 
 *
 */
public class Cursors {

	/**
	 * Wrap an iterator in a cursor.
	 * @param  The type of data to return.
	 * @param iterator An iterator to wrap
	 * @return A cursor returning the elements of the iterator.
	 */
	public static  Cursor wrap(Iterator iterator) {
		return new IteratorCursor(iterator);
	}

	/**
	 * Wrap a collection in a cursor.
	 * @param  The type of data to return.
	 * @param collection A collection to wrap
	 * @return A cursor returning the elements of the collection.
	 */
	public static  Cursor wrap(Collection collection) {
		return new CollectionCursor(collection);
	}

	/**
	 * Filter a cursor.
	 * @param  The type of cursor rows.
	 * @param cursor The source cursor.
	 * @param predicate A predicate indicating which rows to return.
	 * @return A cursor returning all rows for which predicate returns
	 * true.
	 */
	public static  Cursor filter(Cursor cursor, Predicate predicate) {
		return new FilteredCursor(cursor, predicate);
	}

	/**
	 * Create an empty cursor.
	 */
	public static  Cursor empty() {
		return new AbstractCursor() {
			@Override
			public int getRowCount() {
				return 0;
			}

			@Override
			public boolean hasNext() {
				return false;
			}

			@Override
			public T next() {
				throw new NoSuchElementException();
			}
		};
	}

	/**
	 * Read a cursor into a list, closing when it is finished.
	 * @param  The type of item in the cursor.
	 * @param cursor The cursor.
	 * @return A list containing the elements of the cursor.
	 */
	public static  ArrayList makeList(Cursor cursor) {
		ArrayList list;
		try {
			int n = cursor.getRowCount();
			if (n < 0) n = 10;
			list = new ArrayList(n);
			for (T item: cursor) {
				list.add(item);
			}
		} finally {
			cursor.close();
		}

		list.trimToSize();
		return list;
	}

}