问题
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!
回答1:
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;
}
来源：https://stackoverflow.com/questions/26941621/trying-to-use-jfreechart-using-jsp-servles-issues-with-jdbccategorydataset-and