001    /**
002     * ========================================
003     * JFreeReport : a free Java report library
004     * ========================================
005     *
006     * Project Info:  http://reporting.pentaho.org/
007     *
008     * (C) Copyright 2000-2007, by Object Refinery Limited, Pentaho Corporation and Contributors.
009     *
010     * This library is free software; you can redistribute it and/or modify it under the terms
011     * of the GNU Lesser General Public License as published by the Free Software Foundation;
012     * either version 2.1 of the License, or (at your option) any later version.
013     *
014     * This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
015     * without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
016     * See the GNU Lesser General Public License for more details.
017     *
018     * You should have received a copy of the GNU Lesser General Public License along with this
019     * library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
020     * Boston, MA 02111-1307, USA.
021     *
022     * [Java is a trademark or registered trademark of Sun Microsystems, Inc.
023     * in the United States and other countries.]
024     *
025     * ------------
026     * $Id: Function.java 3048 2007-07-28 18:02:42Z tmorgner $
027     * ------------
028     * (C) Copyright 2000-2005, by Object Refinery Limited.
029     * (C) Copyright 2005-2007, by Pentaho Corporation.
030     */
031    
032    package org.jfree.report.expressions;
033    
034    import org.jfree.report.DataSourceException;
035    
036    /**
037     * The interface for report functions.  A report function separates the business
038     * logic from presentation of the result.
039     * <p/>
040     * Since JFreeReport 0.9 functions are considered immutable. During the
041     * advancement process, the function returns a new instance with the updated
042     * state.
043     *
044     * @author Thomas Morgner
045     */
046    public interface Function extends Expression
047    {
048      /**
049       * When the advance method is called, the function is asked to perform the
050       * next step of its computation.
051       * <p/>
052       * The original function must not be altered during that step (or more
053       * correctly, calling advance on the original expression again must not return
054       * a different result).
055       *
056       * @return a copy of the function containing the new state.
057       */
058      public Function advance() throws DataSourceException;
059    }