databasedatarowreader.java

著名的开源仿真软件yale

/*
 *  YALE - Yet Another Learning Environment
 *  Copyright (C) 2002, 2003
 *      Simon Fischer, Ralf Klinkenberg, Ingo Mierswa, 
 *          Katharina Morik, Oliver Ritthoff
 *      Artificial Intelligence Unit
 *      Computer Science Department
 *      University of Dortmund
 *      44221 Dortmund,  Germany
 *  email: [e-mail]
 *  web:   http://yale.cs.uni-dortmund.de/
 *
 *  This program is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU General Public License as 
 *  published by the Free Software Foundation; either version 2 of the
 *  License, or (at your option) any later version. 
 *
 *  This program is distributed in the hope that it will be useful, but
 *  WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 *  General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
 *  USA.
 */
package edu.udo.cs.yale.example;

import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.List;
import edu.udo.cs.yale.example.Example;
import edu.udo.cs.yale.example.Attribute;
import edu.udo.cs.yale.tools.LogService;
import edu.udo.cs.yale.example.ExampleReader;

/**
 * Unlike a {@link FileDataRowReader} that reads examples from a file,
 * objects of this class read examples from a {@link ResultSet}, a data
 * structure that is returned from a database query.
 *
 * @see edu.udo.cs.yale.tools.DatabaseHandler
 * @see edu.udo.cs.yale.operator.DatabaseExampleSource
 * @see edu.udo.cs.yale.operator.KDBExampleSource
 * @version $Id: DatabaseDataRowReader.java,v 2.7 2003/09/10 13:02:06 fischer Exp $
 */
public class DatabaseDataRowReader implements DataRowReader {

    private Attribute[] attributes;
    private ResultSet resultSet;

    private static final int DONT_KNOW_YET = 0;
    private static final int YES           = 1;
    private static final int NO            = 2;

    private int hasNext = DONT_KNOW_YET;

    /** Creates a datarow reader from a ResultSet. The column meta data must be tranformed to an
     *  {@link Attribute} array.
     */
    public DatabaseDataRowReader(Attribute[] attributes,
				 ResultSet resultSet) {
	this.resultSet = resultSet;
	this.attributes = attributes;
    }


    public boolean hasNext() {
	switch (hasNext) {
	case YES: return true;
	case NO: return false;
	case DONT_KNOW_YET:
	    try {
		if (resultSet.next()) {
		    hasNext = YES;
		    return true;
		} else {
		    hasNext = NO;
		    resultSet.close();
		    return false;
		}
	    } catch (SQLException e) {
		LogService.logException("While reading examples from result set:", e);
		return false;
	    }
	default:
	    // impossible
	    return false;
	}
    }

    public DataRow next() {
        if (hasNext()) {
	    hasNext = DONT_KNOW_YET;
	    try {
		return new DatabaseDataRow(resultSet);
	    } catch (SQLException sqle) {
		throw new RuntimeException("Error accessing the result of a query:" + sqle.toString(), sqle);  
	    }
        } else { 
	    return null;  
	}
    } 
}