java JFrame giving me java.lang.NullPointerException and I can't figure out why

Question

Before you mark this post as duplicate and referance What is a NullPointerException, and how do I fix it?, I have read that and (I am still new to java and might not understand this correctly) can't seem to see how that is connected to my error.

I am trying to dispose a java swing JFrame and load another in it's place and have gotten this to work. Even though it is doing exactly what I want, it still gives me the java.lang.NullPointerException error. As far as I understand the code that the error is connected to is functioning as I want it to.

Here is my relevant code. If you need me to include any other code I did not think was necessary please let me know and I will edit.

EDIT

I removed all my previous code and have added mcve code for you.

package test;

public final class Test {

    private static GUI1 gui1;
    private static GUI2 gui2;

    public static void main(String[] args) {
        startGUI1();
    }

    public static void startGUI1() {
        gui1 = new GUI1();
    }

    public static GUI1 getGUI1() {
        return gui1;
    }

    public static void startGUI2() {
        gui2 = new GUI2();
    }

    public static GUI2 getGUI2() {
        return gui2;
    }
}

package test;

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class GUI1 {

    JFrame frame;

    public GUI1() {
        frame = new JFrame();

        frame.setVisible(true);
        frame.setSize(500, 500);
        frame.setLocationRelativeTo(null);
        frame.add(new Pane());
    }

    public class Pane extends JComponent {

        public Pane() {
            try {
                Scanner read = new Scanner(new File("user.txt"));

                if (read.nextInt() != 0) {
                    test.Test.startGUI2();
                    test.Test.getGUI1().frame.dispose();
                }
            } catch (FileNotFoundException ex) {
                Logger.getLogger(GUI1.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }
}

package test;

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class GUI2 {

    JFrame frame;

    public GUI2() {
        frame = new JFrame();

        frame.setVisible(true);
        frame.setSize(500, 500);
        frame.setLocationRelativeTo(null);
        frame.add(new Pane());
    }

    public class Pane extends JComponent {

        JLabel label = new JLabel("GUI2");
        JButton button = new JButton("Start Gui1");

        public Pane() {
        }

        @Override
        protected void paintComponent(Graphics g) {
            Graphics2D g2d = (Graphics2D) g;

            g2d.setColor(new Color(5, 5, 5));
            g2d.fillRect(0, 0, getWidth(), getHeight());

            g2d.setStroke(new BasicStroke(5.0F));
        }
    }
}

this produces the same error

Answer 1

No, your problem is not that the JFrame is null, but rather your Test.gui1 variable is null. I added a line to your code to test it out:

  public Pane() {
     try {
        Scanner read = new Scanner(new File("user.txt"));

        if (read.nextInt() != 0) {
           Test.startGUI2();

           // ********** two lines below added ******
           System.out.println("gui1 null? " + (Test.getGUI1() == null));
           System.out.println("frame null? " + (frame == null));

           Test.getGUI1().frame.dispose();
        }
     } catch (FileNotFoundException ex) {
        Logger.getLogger(GUI1.class.getName()).log(Level.SEVERE, null, ex);
     }

This will return:

gui1 null? true
frame null? false

You're trying to call methods on the gui1 GUI1 object before its constructor has completed, a dangerous thing to do.

But again, the ultimate solution may be different and will depend on what you're trying to achieve. If your goal is to swap views, then I'm with Andrew Thompson -- you should switch JPanel views with a CardLayout.

---

Another way to test that I'm right is to see what happens if we call your frame swapping code in the GUI1 constructor vs giving it a little delay by queuing it on the Swing event thread using SwingUtilities.invokeLater(Runnable). In the code below, try running it with different values for the CREATE_ERROR boolean.

In other words, run it once with

public static final boolean CREATE_ERROR = true; 

and then run it again with

public static final boolean CREATE_ERROR = false; 

and see what happens

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.SwingUtilities;

public class TestA {
   private static GUI1A gui1;
   private static GUI2A gui2;

   public static void main(String[] args) {
      startGUI1();
   }

   public static void startGUI1() {
      gui1 = new GUI1A();
   }

   public static GUI1A getGUI1() {
      return gui1;
   }

   public static void startGUI2() {
      gui2 = new GUI2A();
   }

   public static GUI2A getGUI2() {
      return gui2;
   }
}

class GUI1A {
   public static final boolean CREATE_ERROR = true; 
   private JFrame frame1 = new JFrame("Frame 1");

   public GUI1A() {
      frame1.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      frame1.add(new JLabel("This is Frame 1"));
      frame1.pack();
      frame1.setLocationByPlatform(true);
      frame1.setVisible(true);

      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            if (!CREATE_ERROR) {
               swapFrames();
            }
         }
      });
      if (CREATE_ERROR) {
         swapFrames();
      }
   }

   private void swapFrames() {
      TestA.startGUI2();

      System.out.println("GUI1 is null: " + (TestA.getGUI1() == null));
      System.out.println("frame1 is null: " + (frame1 == null));

      TestA.getGUI1().getFrame().dispose();
   }

   public JFrame getFrame() {
      return frame1;
   }
}

class GUI2A {
   private JFrame frame2 = new JFrame("Frame 2");

   public GUI2A() {
      frame2.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      frame2.add(new JLabel("This is Frame 2"));
      frame2.pack();
      frame2.setLocationByPlatform(true);
      frame2.setVisible(true);
   }
}

---

As a side note, I would rarely if ever swap JFrames like that (don't think I've ever done it come to think of it), although I have shown separate dialog windows. Also, I would take care to do all file reading off of the EDT. For example to take your code, I'd use a SwingWorker:

private class ButtonAction extends AbstractAction {
  public ButtonAction(String name) {
     super(name);
  }

  @Override
  public void actionPerformed(ActionEvent e) {
     final MyWorker myWorker = new MyWorker();
     myWorker.addPropertyChangeListener(new PropertyChangeListener() {

        @Override
        public void propertyChange(PropertyChangeEvent evt) {
           if (evt.getNewValue() == SwingWorker.StateValue.DONE) {
              try {
                 int result = myWorker.get().intValue();
                 if (result != 0) {
                    TestB.startGUI2();
                    System.out.println("gui1 null? " + (TestB.getGUI1() == null));
                    System.out.println("frame null? " + (frame == null));

                    TestB.getGUI1().frame.dispose();
                 }                     
              } catch (InterruptedException | ExecutionException e) {
                 e.printStackTrace();
              }
           }
        }
     });
     myWorker.execute();
  }
}

private class MyWorker extends SwingWorker<Integer, Void> {
  @Override
  protected Integer doInBackground() throws Exception {
     int result = 0;
     Scanner read = null;
     try {
        read = new Scanner(new File(USER_FILE_PATH));
        result = read.nextInt();
     } catch (FileNotFoundException ex) {
        Logger.getLogger(GUI1B.class.getName()).log(Level.SEVERE, null, ex);
     } finally {
        if (read != null) {
           read.close();
        }
     }
     return result;
  }
}

---

The specific reason that gui1 is null at the critical time is because the assignment of an object to it, gui1 = new GUI1(); does not occur until the GUI1 constructor finishes.