Null Pointer Exception still occurring even after passing in the right variable

Question

In the following code I am trying to design a full screen menu. To do this I thought it would be best to use two JPanels. My problem is when trying to switch between the JPanels I get a java.lang.NullPointerException. The exception states that it occurs on line 101 which is this line. m.getMainWindow().getContentPane().setComponentZOrder(m.getOverlay(), 0); Originally I believed to problem to be an instance problem; however, upon passing the menu variable, which is used in the main void at the end of the public class Menu, though all the relevant methods the null pointer still occurs. I would appreciate any help in fixing my problem.

Code

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import net.miginfocom.swing.MigLayout;

public class Menu {
    JFrame myMainWindow = new JFrame("Menu test");
    GridBagConstraints c;
    JLayeredPane lP;
    MyFirstPanel fP;
    MyOverlay oL;
    MigLayout baseLayout;
    MigLayout overlayLayout;

    private void runGUI(Menu m) {       
        myMainWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        myMainWindow.setLayout(new GridBagLayout());
        setContraints();
        createPanels(m);
        myMainWindow.getContentPane().add(fP, c);
        myMainWindow.getContentPane().add(oL, c);
        myMainWindow.pack();
        myMainWindow.setVisible(true);
    }

    private void setContraints() {
        c = new GridBagConstraints();
        c.gridx = 0;
        c.gridy = 0;
        c.weightx = 1;
        c.weighty = 1;
        c.fill = GridBagConstraints.BOTH;
    }

    private void createPanels(Menu m) {
        createFirstPanel(m);
        createOverlay();
    }

    private void createFirstPanel(Menu m) {
        baseLayout = new MigLayout("", "", "");
        fP = new MyFirstPanel(baseLayout, m);

    }

    private void createOverlay() {
        overlayLayout = new MigLayout("", "", "");
        oL = new MyOverlay(overlayLayout, true);

    }

    public JPanel getOverlay() {
        return oL;
    }

    public JFrame getMainWindow() {
        return myMainWindow;
    }

    public static void main(String[] args) {
        Menu menu = new Menu();
        menu.runGUI(menu);
    }
}

class MyFirstPanel extends JPanel {
    MyFirstPanel(MigLayout layout, Menu mM) {
        setLayout(layout);
        setBackground(Color.RED);
        setPreferredSize(new Dimension(600, 600));
        add(new JLabel("hiasdfadsf"), "wrap");
        JButton jb = new JButton("Press Me");
        jb.addActionListener(new CustomActionListener(mM));
        add(jb);
    }
}

class MyOverlay extends JPanel {
    MyOverlay(MigLayout layout, boolean visible) {
        setBackground(Color.GREEN);
        setPreferredSize(new Dimension(600, 600));
        add(new JLabel("hi"));

        setVisible(visible);
    }
}

class CustomActionListener implements ActionListener {
    Menu m;

    public CustomActionListener(Menu mM) {
        this.m = mM;
        m.getMainWindow().getContentPane().setComponentZOrder(m.getOverlay(), 0);
    }

    public void actionPerformed(ActionEvent e) {
        m.getMainWindow().getContentPane().setComponentZOrder(m.getOverlay(), 0);
    }
}

Exception

Exception in thread "main" java.lang.NullPointerException
    at java.awt.Container.setComponentZOrder(Container.java:760)
    at packages.CustomActionListener.<init>(Menu.java:101)
    at packages.MyFirstPanel.<init>(Menu.java:75)
    at packages.Menu.createFirstPanel(Menu.java:44)
    at packages.Menu.createPanels(Menu.java:38)
    at packages.Menu.runGUI(Menu.java:21)
    at packages.Menu.main(Menu.java:64)

Possible duplicate of [What is a NullPointerException, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — Frakcool, Jul 03 '16 at 19:03
Don't try to play with the ZOrder. If you want to swap panels then use a [Card Layout](http://docs.oracle.com/javase/tutorial/uiswing/layout/card.html). Also, when you post the code should only contain JDK classes. MigLayout is not part of the JDK. People can't test your code. — camickr, Jul 03 '16 at 19:17
@camickr Thank you for the info and I will leave mig layout out next time — Dan, Jul 03 '16 at 19:18

score 3 · Accepted Answer · answered Jul 03 '16 at 19:05

3

The method getOverlay() returns null. The overlay it returns is created in

private void createPanels(Menu m) {
    createFirstPanel(m);
    createOverlay();
}

So the overlay is created after the first panel. But the creation of the first panel needs the overlay to exist. Inverting those two instructions should fix the problem.

answered Jul 03 '16 at 19:05

JB Nizet

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In doing this a `java.lang.IllegalArgumentException: component and container should be in the same top-level window` is created – Dan Jul 03 '16 at 19:07
That's another matter. Try understanding what it means by examining the stack trace, reading the javadoc, etc. If you're stuck, ask another question. – JB Nizet Jul 03 '16 at 19:12
No Problem. Will do that – Dan Jul 03 '16 at 19:13

Null Pointer Exception still occurring even after passing in the right variable

1 Answers1