How to return DefaultCategoryDataset

Question

Hi all ,
I'm trying to return a DefaultCategoryDatased filled from database but i'm not sure the method is correct.. Here is my code:
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.logging.Level;
import java.util.logging.Logger;
import org.jfree.data.category.DefaultCategoryDataset;
* @author sasho_b
public class Charts {
    String host = "jdbc:derby:release\\snejana_bah";
    String uName = "fester";
    String uPass = "fester";
    DefaultCategoryDataset dataset;
    Double limit;
    Double total;
    String category;
    public DefaultCategoryDataset getChartDataLimit() {
        try {
            Connection con = DriverManager.getConnection(host, uName, uPass);
            Class.forName("org.apache.derby.jdbc.EmbeddedDriver");
            PreparedStatement ps = con.prepareStatement("select sum(t.AMOUNT) as total,t.PRODUCT_NAME as category, l.\"LIMIT\" as lim from fester.transactions t, fester.LIMITS l \n"
                    + "where l.CAT_ID = t.CATEGORY\n"
                    + "group by t.PRODUCT_NAME, l.\"LIMIT\"");
            ResultSet rs = ps.executeQuery();
            while (rs.next()) {
                total = rs.getDouble("total");
                category = rs.getString("category");
                limit = rs.getDouble("lim");
                dataset.setValue(total, category, limit);
        } catch (SQLException | ClassNotFoundException ex) {
            Logger.getLogger(Reports.class
                    .getName()).log(Level.SEVERE, null, ex);
        return dataset;
}I refer to this method in a button action listener. I'm just trying to set DefaultCategoryDataset and pass it to a JFreeChart. The error is at the line:
dataset.setValue(total, category, limit);with message:
java.lang.NullPointerExceptionI'm sure the query works fine...
Any ideas?
Thanks in advance,
Alexander.

Answer 1

>
Thanks for reply - it works. But... this returns the last row from DB. Can i fetch all the rows to the dataset ?
>
One reason it returns the last row is because the code provided is creating a new instance each time through the loop
while (rs.next()) {
    dataset = new DefaultCategoryDataset(); // <-------        
    total = rs.getDouble("total");
    category = rs.getString("category");
    limit = rs.getDouble("lim");               
    dataset.setValue(total, category, limit);
}Create the dataset BEFORE the loop
-- create the dataset BEFORE the loop
dataset = new DefaultCategoryDataset(); // <-------        
while (rs.next()) {
    total = rs.getDouble("total");
    category = rs.getString("category");
    limit = rs.getDouble("lim");               
    dataset.setValue(total, category, limit);
}Also the 'setValue' method will NOT add a new row to the dataset unless the 'rowKey' value you provide is different. If you provide the same value for 'rowKey' in the 'setValue' method you will replace the prior 'rowKey'/'columnKey' value with the new one.
See the Dataset API
http://www.jfree.org/jfreechart/api/javadoc/org/jfree/data/category/DefaultCategoryDataset.html
>
void setValue(double value, java.lang.Comparable rowKey, java.lang.Comparable columnKey)
Adds or updates a value in the table and sends a DatasetChangeEvent to all registered listeners.
void setValue(java.lang.Number value, java.lang.Comparable rowKey, java.lang.Comparable columnKey)
Adds or updates a value in the table and sends a DatasetChangeEvent to all registered listeners.