Issue with getting text from a text field with .getText()

Question

I am building a application that verifies a logins from a Amazon Web Service s3 file, It can verify a login fine, but the issue arises when I try to get the text entered by the user in a textField.

This is the section of code that builds the UI :

public class UserInterface extends JFrame implements ActionListener {

JLayeredPane pane;
JFrame f;
JTextField usernameLogin;
JTextField passwordLogin;
UserInterface ui;
JButton loginButton;
static String loggedInAs = null;
static AmazonS3       s3;
static boolean tryToLogin = false;


public UserInterface() {

    JLayeredPane pane = new JLayeredPane();

    JTextField usernameLogin = new JTextField("Username...",20);
    usernameLogin.setLocation(650,200);
    usernameLogin.setSize(500, 30);
    usernameLogin.setVisible(true);
    pane.add(usernameLogin, 1, 0);

    JTextField passwordLogin = new JTextField("Password...",20);
    passwordLogin.setLocation(650,240);
    passwordLogin.setSize(500, 30);
    passwordLogin.setVisible(true);
    pane.add(passwordLogin, 1, 0);

    JButton loginButton = new JButton("login");
    loginButton.setLocation(650,290);
    loginButton.setSize(75, 20);
    loginButton.addActionListener(this);
    loginButton.setVisible(true);
    pane.add(loginButton, 1, 0);


    this.add(pane);

}

I dont think the problom comes from there, but I could be wrong. The next part of the program is a logic processor that works with the server to verify a login with Amazon s3. It is in the main().

        int INFINITE = 1;
    try {
        System.out.println("Downloading an object");
        S3Object object = s3.getObject(new GetObjectRequest("saucyMMO", "logins.txt"));
        System.out.println("Content-Type: "  + object.getObjectMetadata().getContentType());

        while (INFINITE == 1) {
            System.out.println("ran");
            if (tryToLogin == true) {
                System.out.println("ran2");
                INFINITE = 0;
             BufferedReader br = new BufferedReader(new InputStreamReader(object.getObjectContent()));
             String lineValue = null;
             while((lineValue = br.readLine()) != null && loggedInAs == null){
                 String splitResult[] = lineValue.split(",");
                 boolean retVal = splitResult[0].equals(ui.usernameLogin.getText());
                 boolean retVal2 = splitResult[1].equals(ui.passwordLogin.getText());
               if (retVal == true && retVal2 == true) {
                loggedInAs = splitResult[0];
                System.out.println("logged in as : " + loggedInAs);
               }
               else {
                    System.out.println("SPLIT 0 : " + splitResult[0]);
                    System.out.println("SPLIT 1 : " + splitResult[1]);
               }
             }
             }
        }
    } catch (AmazonServiceException ase) {

    } catch (AmazonClientException ace) {

    } catch (IOException e) {
        e.printStackTrace();
    } 

I always get a null pointer exception when I call the "ui.usernameLogin.getText()" or "ui.passwordLogin.getText()".

Specifically,

java.lang.NullPointerException
at UserInterface.main(UserInterface.java:102)

Answer 1

You are defining a local variable here in constructor

JTextField usernameLogin = new JTextField("Username...",20);

that is different from JTextField usernameLogin; declaration this is never initialized and is null that's why you have a NullPointerException .

Change this

public class UserInterface extends JFrame implements ActionListener {
  JLayeredPane pane;
  JFrame f;
  JTextField usernameLogin;
  JTextField passwordLogin;
  UserInterface ui;
  JButton loginButton;   


  public UserInterface() {
    LayeredPane pane = new JLayeredPane();

    JTextField usernameLogin = new JTextField("Username...",20);
    usernameLogin.setLocation(650,200);
    usernameLogin.setSize(500, 30);
    usernameLogin.setVisible(true);
    pane.add(usernameLogin, 1, 0);

    JTextField passwordLogin = new JTextField("Password...",20);
    passwordLogin.setLocation(650,240);
    passwordLogin.setSize(500, 30);
    passwordLogin.setVisible(true);
    pane.add(passwordLogin, 1, 0);

    JButton loginButton = new JButton("login");
    loginButton.setLocation(650,290);
    loginButton.setSize(75, 20);
    loginButton.addActionListener(this);
    loginButton.setVisible(true);
    pane.add(loginButton, 1, 0);


    this.add(pane);
}

TO

public class UserInterface {
    private JLayeredPane pane;
    private JTextField usernameLogin;
    private JTextField passwordLogin;
    private JButton loginButton; 
    private JTextField usernameLogin; 
    private JFrame frame;

    public UserInterface() {
    usernameLogin = new JTextField("Username...",20);
    usernameLogin.setLocation(650,200);
    usernameLogin.setSize(500, 30);
    usernameLogin.setVisible(true);
    pane.add(usernameLogin, 1, 0);

    passwordLogin = new JTextField("Password...",20);
    passwordLogin.setLocation(650,240);
    passwordLogin.setSize(500, 30);
    passwordLogin.setVisible(true);
    pane.add(passwordLogin, 1, 0);

    loginButton = new JButton("login");
    loginButton.setLocation(650,290);
    loginButton.setSize(75, 20);
    loginButton.addActionListener(this);
    loginButton.setVisible(true);
    pane.add(loginButton, 1, 0);

    frame= new JFrame();
    //configure JFrame

    frame.add(pane);
    frame.pack();//size the frame
    frame.setVisible(Boolean.TRUE);
    }

      //you have to added to some component
      private class MyActionListener implements ActionListener{
             @Override
             public void actionPerformed(ActionEvent evt){
                    //code here
             }
      }

}

A few advices :

Don't extends JFrame have a reference instead (composition over inheritance) and don't implement in top classes ActionListener instead use Anonymous class or Inner Classes

Some coding :

(retVal == true && retVal2 == true) change it to (retVal && retVal2)

Manage exceptions , never blank catches

} catch (AmazonServiceException ase) {

    } 

Answer 2

From the code you supplied, you never assigned an object to the ui variable. Additionally, usernameLogin was created locally, and not assigned to the class's member variable

Answer 3

That means that you are trying to pass a null value into somewhere where it doesn't take a null value. You will need to do a bit of error handling as to check for a null value. Also you use the == operator to check for null values. The .equals compares objects.

    if(ui.username.getText() != null){
          boolean retVal = splitResult[0].equals(ui.usernameLogin.getText());
          boolean retVal2 = splitResult[1].equals(ui.passwordLogin.getText());
    }