Question

I want to insert values to my database table made in ms access. This code works before and I can’t understand why it gets some error now.

Exception in thread "main" java.lang.NullPointerException
at storeapp.Trylog.<init>(Trylog.java:28)
at storeapp.Trylog.main(Trylog.java:45)

I believe I configured the driver correctly.

I have a table name ACCOUNT with columns Username and Password..

And here is the code…..

public class Trylog extends JFrame{
Connection con;
Statement st;
ResultSet rs;

public Trylog(){
    connect();

    String u = "Katherine";
    String p = "kat";

    String sql = "Insert into ACCOUNT(Username, Password) Values ('"+u+"' , '"+p+"' ) ";
    try {
        rs = st.executeQuery(sql);
    } catch (SQLException ex) {
    }
}

public void connect(){
    try {
            String driver = "sun.jdbc.odbc.JdbcOdbcDriver";
             Class.forName(driver);
             String db = "jdbc:odbc:access";
             con = DriverManager.getConnection(db);
             st = con.createStatement();
    }
    catch (Exception ex){}
}

public static void main(String[] args) {
    Trylog r = new Trylog();
    r.setVisible(true);
    r.setSize(600, 800);
    r.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

}

}

Do anyone know why I get such error?

This is the line that cause the exception base from line 28 and 46

rs = st.executeQuery(sql);

and

 Trylog r = new Trylog();

It prints

Exception in thread "main" java.lang.RuntimeException: java.sql.SQLException: No ResultSet was produced

Caused by: java.sql.SQLException: No ResultSet was produced

Answer 1

Your concrete problem is caused by bad exception handling (even more, there’s no exception handling at all). You’ve there catch blocks which completely swallows the exception, such as here:

catch (Exception ex){}

This way the code continues to run in case of an exception, possibly leaving the variables in the try block such as st uninitialized. After that, those uninitialized (i.e. they are still null) variables are been accessed which in turn causes the NullPointerException.

This shouldn’t happen. When an exception is caught, the code should properly deal with it. In this case, it should stop immediately and show the error detail in some way. At this moment, easiest is to rethrow it as a runtime exception so that you will at least get all the necessary information about the problem:

catch (Exception ex) { 
    throw new RuntimeException(ex);
}

(you can always improve this by logging the stacktrace and showing some user friendly message dialog and then exiting the program, for example)

The thrown exception is basically the whole answer to the root cause of your problem. The art is to be able to interpret and understand the stacktrace.

---

Update as per your update, you got

java.sql.SQLException: No ResultSet was produced

See, there’s your answer! There’s no ResultSet been produced at all for the given query, while you expected it to be produced. And indeed, an INSERT query won’t return a result set. It will at highest return the amount of affected rows.

You need to replace

rs = st.executeQuery(sql);

by

st.executeUpdate(sql);

See also the executeQuery() javadoc and the executeUpdate() javadoc. They precisely tell for which SQL statements the method should be used.

executeQuery

Executes the given SQL statement, which returns a single ResultSet object.

Parameters:
sql – an SQL statement to be sent to the database, typically a static SQL SELECT statement

executeUpdate

Executes the given SQL statement, which may be an INSERT, UPDATE, or DELETE statement or an SQL statement that returns nothing, such as an SQL DDL statement.

Parameters:
sql – an SQL Data Manipulation Language (DML) statement, such as INSERT, UPDATE or DELETE; or an SQL statement that returns nothing, such as a DDL statement.