尝试通过 JSP/Servles 使用 JFreeChart；JDBCCategoryDataset 和 CategoryDataset 的问题 [英] Trying to use JFreeChart using JSP/Servles; issues with JDBCCategoryDataset and CategoryDataset

问题描述

我正在尝试连接到数据库并在 servlet 中执行查询.我正在关注这个例子 JFreeChart Example.如果您查看 readData() 方法，当它最初是 JDBCCategoryDataset 时，它返回一个 CategoryDataset.我收到一个错误，直到我将它转换为 CategoryDataset.当我运行代码时，它不起作用，告诉我它无法投射.任何帮助将不胜感激！

i am trying to connect to a database and execute a query in a servlet. I am following this example JFreeChart Example. If you look at the readData() method, it returns a CategoryDataset when it originally was a JDBCCategoryDataset. I get an error until i cast it do a CategoryDataset. When i run the code, it doesn't work, telling me it cannot cast. Any help would be much appreciated!

推荐答案

因为 JDBCCategoryDataset 实现了CategoryDataset 接口，赋值时不需要强制转换:CategoryDataset data = readData(); 我从下面概述的 readData() 的变体中得到以下图表.我怀疑你有另一个问题.

Because JDBCCategoryDataset implements the CategoryDataset interface, no cast should be required in the assignment: CategoryDataset data = readData(); I get the following chart from the variation of readData() outlined below. I suspect that you have another problem.

private CategoryDataset readData() { 

    JDBCCategoryDataset data = null; 
    Connection con; 
     try { 
        con = DriverManager.getConnection("jdbc:h2:mem:test", "", ""); 
        data = new JDBCCategoryDataset(con); 
        String sql = "select TYPE_NAME, PRECISION "
            + "from INFORMATION_SCHEMA.TYPE_INFO "
            + "where PRECISION BETWEEN 1 AND 12"; 
        data.executeQuery(sql); 
        con.close(); 
    } 
    … 
    return data; 
 } 

这篇关于尝试通过 JSP/Servles 使用 JFreeChart；JDBCCategoryDataset 和 CategoryDataset 的问题的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！