Question

I’m getting a NullPointerException error at line 77 lblNewLabel.setVisible(false);
which is called from line 65 runTest(); in the following code. (This is a dummy project I wrote to simulate a problem I’m having in a larger project). What I’m trying to do is change the attribute of several fields, buttons, etc based on user action at various places in the project. I would like to group all the changes in a separate method that can be called from various other methods. I’m still a Java novice, having come from some Visual Basic and Pascal experience. Seems like what I’m trying to do should be straight forward, but for now, I’m at a loss. Thanks in advance for your suggestions.

package woodruff;

import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.JLabel;
import javax.swing.JButton;
import javax.swing.SwingConstants;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JTextField;
import java.awt.event.FocusAdapter;
import java.awt.event.FocusEvent;

public class MyTest extends JFrame {

    private JPanel contentPane;
    private JTextField txtHasFocus;
    private JLabel lblNewLabel;

    /**
     * Create the frame.
     */
    public MyTest() {
        initialize();
    }

    private void initialize() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 237, 161);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(contentPane);
        contentPane.setLayout(null);
        final JLabel lblNewLabel = new JLabel("This is a label.");
        lblNewLabel.setHorizontalAlignment(SwingConstants.CENTER);
        lblNewLabel.setHorizontalTextPosition(SwingConstants.CENTER);
        lblNewLabel.setBounds(10, 25, 202, 14);
        contentPane.add(lblNewLabel);

        JButton btnShow = new JButton("Show");
        btnShow.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {
                lblNewLabel.setVisible(true);
            }
        });
        btnShow.setBounds(10, 50, 89, 23);
        contentPane.add(btnShow);

        JButton btnHide = new JButton("Hide");
        btnHide.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                lblNewLabel.setVisible(false);
            }
        });
        btnHide.setBounds(123, 50, 89, 23);
        contentPane.add(btnHide);

        txtHasFocus = new JTextField();
        txtHasFocus.addFocusListener(new FocusAdapter() {
            @Override
            public void focusGained(FocusEvent arg0) {
                // Following results in NullPointerException error
                // at woodruff.MyTest.runTest(MyTest.java:77)
                runTest();
            }
        });
        txtHasFocus.setHorizontalAlignment(SwingConstants.CENTER);
        txtHasFocus.setText("Has Focus?");
        txtHasFocus.setBounds(67, 92, 86, 20);
        contentPane.add(txtHasFocus);
        txtHasFocus.setColumns(10);
    }

    private void runTest() {
        lblNewLabel.setVisible(false);
    }

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    MyTest frame = new MyTest();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
}

Answer 1

In the initialize() method, you have created a local variable for JLabel, and hence are not initializing the instance field, as a reason it remains initialized to null, and hence NPE.

final JLabel lblNewLabel = new JLabel("This is a label.");

change the above line to: –

lblNewLabel = new JLabel("This is a label.");