GUI and NullPointerException

[Humphrey Bogart]

Hello. I'm working on a project that uses a GUI to show and manipulate entries in a database. The code compiles and runs, and the GUI correctly shows the first entry in the database. I then added a JButton to the GUI so the user can advance to the next entry in the database, and wrote the code for the "Next" button. The program compiles and runs, and the GUI correctly shows the first entry in the database. But when I click on the "Next" button, I get a NullPointerException. Can anyone help me fix this?

The code to show the first entry in the database:

package Employees;
// This program builds a GUI to access and manipulate the "Employess" database
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;
import java.sql.ResultSet;
import javax.swing.JOptionPane;

public class Workers extends javax.swing.JFrame {
    Connection con;
    ResultSet rs;
    Statement stmt;

    public Workers() {
        initComponents();
        DoConnect();     
    }
    
    public void DoConnect() {
        try {
            // Connecting to the database
            String host = "jdbc:derby://localhost:1527/Employees";
            String uName = "admin";
            String pWord = "admin";
            Connection con = DriverManager.getConnection(host, uName, pWord);
            
            // Query SQL and load results into ResultSet
            Statement stmt = con.createStatement();
            String SQL = "SELECT * FROM APP.Workers";   
            ResultSet rs = stmt.executeQuery(SQL);
            
            // Getting the data from the database
            rs.next();
            int id_col = rs.getInt("ID");
            String id = Integer.toString(id_col);
            String first_name = rs.getString("First_Name");
            String last_name = rs.getString("Last_Name");
            String job = rs.getString("Job_Title");
            
            //Display the first recod in the GUI
            textID.setText(id);
            textFName.setText(first_name);
            textLName.setText(last_name);
            textJob.setText(job);
            
        }        
        catch (SQLException err) {
            JOptionPane.showMessageDialog(Workers.this, err.getMessage());
            
        }
    }

The code to show the "Next" entry in the database. The NullPointerException points to the first line of the "try" block: if (rs.next()) {

private void butNextActionPerformed(java.awt.event.ActionEvent evt) {
        
        try {
            if ( rs.next() ){
                int id_col = rs.getInt("ID");
                String id = Integer.toString(id_col);
                String first_name = rs.getString("First_Name");
                String last_name = rs.getString("Last_Name");
                String job = rs.getString("Job_Title");
            
                textID.setText(id);
                textFName.setText(first_name);
                textLName.setText(last_name);
                textJob.setText(job);
                
            }
            else {
                rs.previous();
                JOptionPane.showMessageDialog(Workers.this, "No More Records in File");
                
            }        
        }     
        
        catch (SQLException err) {
            JOptionPane.showMessageDialog(Workers.this, err.getMessage());
        }  
    
    }

Thanks for your help.

[pbrockway2]

A NullPointerException occurs when you use an expression as if it had a nonnull value when it is really null. Arrays and method calls are common places for this to happen, and the thing which is null can be a variable or a more complex expression.

String[] arr;
System.out.println(arr[0]); // bad! arr is null
arr = new String[42];
System.out.println(arr[0]); //ok
System.out.println(arr[0].length()); // bad! arr[0] is null

Your case is comparatively simple:

rs.next()

There's only one thing that can possibly be null here: and that's rs. (If the NullPointerExpression occurs somewhere within next() instead, the compiler will say that.) We can easily check that using System.out.println().

try {
    System.out.println("About to call next() in the button handler, rs=" + rs);
    if ( rs.next() ){
        int id_col = rs.getInt("ID");
        // etc

Once you have verified that rs is the culprit here (that it is null) then you have to go back through your code to where you thought you had given rs a nonnull value and figure out why that didn't happen.

[Edit] More generally, it might be the case that you gave rs a nonnull value and it became null later. But this is not the case here - you assign a value to a ResultSet variable exactly once. Variables in Java don't just become null, you have to assign null to them. Evidently that assignment to a ResultSet variable did not do what you thought it did.

[Humphrey Bogart]

Hello. Thanks for your reply.

Sorry: line 2 in your second eample looks like it was cut off (at least on my screen).
System.out.println("About to call next() in the button handler, rs = " + (this part is missing))

I'm guessing the missing part is rs.next(), which if I understand correctly should return either true or false.

[pbrockway2]

[Edit] Ignore what was here.

The line should say

System.out.println(
        "About to call next() in the button handler, rs=" 
        + rs);

What I'm thinking you'll see is a message "About to call next() in the button handler, rs=null".

[Humphrey Bogart]

Hello. So I tried this:
[CODE]
private void butNextActionPerformed(java.awt.event.ActionEvent evt) {

try {
System.out.println("trying Next button: rs = " + rs);
if ( rs.next() ){
int id_col = rs.getInt("ID");
String id = Integer.toString(id_col);
//etc
[\CODE]

The program dutifully compiles, runs, and correctly shows the first entry in the database. When I click the next button, the system prints out "trying Next button rs = null", and then the npe error messages.

So, I have only used rs in three lines of code (see initial post): line 12, line 34, and then in the if (rs.nest()) { line. I have no idea how rs then gets set to null. Yes, I am using NetBeans, so I left out the parts of the code that NetBeans creats automatically. I will look through that code to see if there are any references to rs there.

Thanks for your help.

[pbrockway2]

As I suggested it is assignments to rs that matter. You say things like rs.getInt("ID"), but there's no way that calling the nextInt() is going to make rs null.

The only assignment occurs where you say:

ResultSet rs = stmt.executeQuery(SQL);

You can System.out.println(rs) at this point and see that it has a perfectly good, nonnull value. As mentioned before, there is nothing in the code that assigns any different value. Yet rs is null in the event handler as you've found.

There can only be one way out of the paradox: the rs variable you give a nonnull value to in DoConnect() is not the same variable as the rs variable you use in the button handler. If that's not enough of a hint, remember that you get a new variable each and every time you declare one. And a declaration is a thing that gives the variable and its type.

[Humphrey Bogart]

Hello. I appreciate the help, and I confess I'm still not getting it.

As you say, after the line rs = stmt.executeQuery(SQL), I can get the system to tell me rs = jdbc derby client localhost etc.
And after the try { if (rs.next()) { line, the system tells me rs = null

(I am learning some useful tips on tracking down problems!!)

I notice the Next button ActionPerformed is private, so I tried to re-assign rs, like so:

private void butNextActionPerformed(java.awt.event.ActionEvent evt) {
                
        try {
            Statement stmt = con.createStatement();
            String SQL = "SELECT * FROM APP.Workers";   
            ResultSet rs = stmt.executeQuery(SQL);
            
            System.out.println("rs3 = " + rs);
            
            if ( rs.next() ){
                int id_col = rs.getInt("ID");
                String id = Integer.toString(id_col);
                String first_name = rs.getString("First_Name");
                String last_name = rs.getString("Last_Name");
                String job = rs.getString("Job_Title");
            
                textID.setText(id);
                textFName.setText(first_name);
                textLName.setText(last_name);
                textJob.setText(job);
                
            }
            else {
                rs.previous();
                JOptionPane.showMessageDialog(Workers.this, "No more records in file");
                
            }        
        }     
        
        catch (SQLException err) {
            JOptionPane.showMessageDialog(Workers.this, err.getMessage());
        }  
    
    }

The code compiles, runs, the GUI correctly displays the first entry in the database, and when I click the next buton, I get npe, this time referencing the line Statement stmt = con.createStatement();

Can I get another hint? Sorry to be so obtuse.

[Fubarable]

There's only one variable on that line that can cause a NPE, con. Have you tested if it's null before using it? Where do you create it?

[Fubarable]

Ah, you're shadowing a variable. For instance:

class Foo {
   private Bar bar; // Line (A)

   public Foo() {
      Bar bar = new Bar(); // Line (B)
   }

   public void someMethod() {
      bar.someBarMethod();  // Line (C)
   }

The code above will cause a NPE to occur at line (C). The reason is because even though you declare your Bar variable, bar, in the class at line (A), you re-declare it in the constructor and create it there on line (B). This re-declared bar is local to the constructor, doesn't exist outside of the block it was declared in, and has no effect on the bar variable declared at line (A). The solution is not to re-declare bar locally, to instead use the class variable:

class Foo {
   private Bar bar; // Line (A)

   public Foo() {
      // Bar bar = new Bar(); // Line (B-1)
      bar = new Bar(); // Line (B-2)
   }

   public void someMethod() {
      bar.someBarMethod();  // Line (C)
   }

Now (C) will not throw a NPE.

[Humphrey Bogart]

Hello. thanks for your response.

Bu..bu..but..

In my inital post (above) I presented some code that did not have this kind of variable shadowing (as far as I understand it), yet the program was giving me NPE only when I clicked on the Next button in the GUI. If I understand pbrockway2 correctly, s/he was suggesting that the rs variable in DoConnect() is different from the rs variable in butNextActionPerformed().

So I changed the code as a clumsy way to force rs to be assigned as it is in DoConnect(). Anyways, it doesn't work.

I appreciate any ideas you have to fix this. thanks.

[pbrockway2]

In your original DoConnect() you were also shadowing rs.

You declared it as part of the class near the start, then you declared another rs in DoConnect(). The first (class) declaration is the good one because you want to use this variable in two methodz. Remove the declaration in DoConnect().

[Humphrey Bogart]

Hello. Eureka - not only does it work, but I understand better the entire issue of rs variable being assinged at one point but null at another! As usual, the cause of 98% of my problems can be found between my ears.

Thank you so much.

OK now, I'm going to build the "First", "Back", and "Last" buttons to move around in the database. yippee

[pbrockway2]

You're welcome. It's worth the pain once, to really see what's going on.

Good luck with the rest of the program.