I’m trying to get text from a text field but every time I run the program I get a NullPointerException. Similar problems I’ve found on the internet involve the text field being redeclared as a local variable, but I can’t see that I am doing that here.
The error is :
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at the line specified. Any help would be much appreciated
EDIT:
This short program demonstrates the problem
import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
public class Test{
public static final Test application = new Test();
private JFrame jFrame = null;
private JPanel jContentPane = null, buttonsPanel = null, cardPanel = null, manualPanel = null, uploadPanel = null,
defaultPanel = null;
private JPanel[] mainPanel = {getDefaultPanel(), getManualPanel(), getUploadPanel()};
private JButton manualButton = null, uploadButton = null, manualAssignButton = null;
private JTextField manualEntryField = null;
private JLabel manualLabel = null;
final static String DEFAULTPANEL = "Default";
final static String MANUALPANEL = "Manual";
final static String UPLOADPANEL = "Upload";
/**
* Main method
* @param args
*/
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
application.getJFrame().setVisible(true);
}
});
}
/**
* Get the main application frame
* @return JFrame
*/
private JFrame getJFrame() {
if (jFrame == null) {
jFrame = new JFrame();
jFrame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
jFrame.setResizable(false);
// Add the main content pane
jFrame.setContentPane(getJContentPanel());
jFrame.addWindowListener(new java.awt.event.WindowAdapter() {
public void windowClosing(java.awt.event.WindowEvent e) {
closeApplication();
}
});
}
jFrame.pack();
jFrame.setLocationRelativeTo(null);
return jFrame;
}
/**
* Get main content
* @return JPanel
*/
private JPanel getJContentPanel(){
if (jContentPane == null) {
jContentPane = new JPanel();
jContentPane.setPreferredSize(new Dimension(500, 200));
jContentPane.setLayout(new BorderLayout());
jContentPane.add(getButtonsPanel(), BorderLayout.NORTH);
jContentPane.add(getCardPanel(), BorderLayout.CENTER);
}
return jContentPane;
}
private JPanel getButtonsPanel() {
if(buttonsPanel == null){
buttonsPanel = new JPanel();
buttonsPanel.setPreferredSize(new Dimension(500, 50));
buttonsPanel.setBorder(BorderFactory.createLineBorder(Color.gray));
manualButton = new JButton("Enter manually");
manualButton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
CardLayout cl = (CardLayout)(cardPanel.getLayout());
cl.show(cardPanel, MANUALPANEL);
}
});
uploadButton = new JButton("Upload file");
uploadButton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
CardLayout cl = (CardLayout)(cardPanel.getLayout());
cl.show(cardPanel, UPLOADPANEL);
}
});
buttonsPanel.add(manualButton);
buttonsPanel.add(uploadButton);
}
return buttonsPanel;
}
private JPanel getCardPanel() {
if(cardPanel == null){
cardPanel = new JPanel(new CardLayout());
cardPanel.setPreferredSize(new Dimension(500, 200));
cardPanel.setBorder(BorderFactory.createLineBorder(Color.gray));
cardPanel.add(mainPanel[0], DEFAULTPANEL);
cardPanel.add(mainPanel[1], MANUALPANEL);
cardPanel.add(mainPanel[2], UPLOADPANEL);
}
return cardPanel;
}
private JPanel getDefaultPanel() {
if(defaultPanel == null){
defaultPanel = new JPanel();
}
return defaultPanel;
}
private JPanel getManualPanel() {
if(manualPanel == null){
manualPanel = new JPanel();
manualPanel.setLayout(new FlowLayout());
manualEntryField = new JTextField(10);
manualEntryField.setText("Enter code");
manualAssignButton = new JButton("Assign");
manualAssignButton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
// Here is the exception
System.out.println(manualEntryField.getText().trim());
}
});
manualLabel = new JLabel("Please enter a code and press assign");
manualPanel.add(manualEntryField);
manualPanel.add(manualAssignButton);
manualPanel.add(manualLabel);
}
return manualPanel;
}
private JPanel getUploadPanel() {
if(uploadPanel == null){
uploadPanel = new JPanel();
}
return uploadPanel;
}
private void closeApplication() {
System.exit(0);
}
}
After looking at your SSCCE, I managed to solve the NPE by commenting out the redundant
Testthat was instantiated, and creating the panel array in the constructor. E.G.Note: I cannot quite explain why the changes solved the NPE, I was just refactoring ‘suspicious’ code.