在Netbeans中使用JFree绘制折线图时出现java未使用的导入错误

Question

我需要使用Netbeans中的数据库值绘制一个折线图。目前，我收到一个未使用的导入错误：

unused import java.sql.*

我不知道为什么，但是如果我删除import语句，我会得到以下错误，并且我的图形不会生成

Exception in thread "main" java.lang.NullPointerException
    at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1008) at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1008)
    at java.lang.Double.valueOf(Double.java:504)
    at org.jfree.data.jdbc.JDBCCategoryDataset.executeQuery(JDBCCategoryDataset.java:276)
    at org.jfree.data.jdbc.JDBCCategoryDataset.executeQuery(JDBCCategoryDataset.java:191)
    at newchart4.NewChart4.main(NewChart4.java:18)
Java Result: 1
BUILD SUCCESSFUL (total time: 22 seconds)

这是我的代码：

import java.sql.*;
import org.jfree.ui.*;
import org.jfree.chart.ChartPanel;
import org.jfree.chart.ChartFactory;
import org.jfree.chart.JFreeChart;
import org.jfree.chart.plot.PlotOrientation;
import org.jfree.data.jdbc.JDBCCategoryDataset;

public class NewChart4 {
    public static void main(String[] args) throws Exception {
        String query = "SELECT * from session_details_table";
        JDBCCategoryDataset dataset = new JDBCCategoryDataset(
            "jdbc:mysql://localhost:3306/kumararaja",
            "com.mysql.jdbc.Driver",
            "root",
            "test123"
        );
        dataset.executeQuery(query);

        JFreeChart chart = ChartFactory.createLineChart(
            "u_id", "s_id", "cpm",
            dataset, PlotOrientation.VERTICAL, true, true, false
        );

        ChartPanel chartPanel = new ChartPanel(chart);
        chartPanel.setPreferredSize(new java.awt.Dimension(500, 270));

        ApplicationFrame f = new ApplicationFrame("Chart");
        f.setContentPane(chartPanel);
        f.pack();
        f.setVisible(true);
    }
}