Need help with addActionListener

Hi everyone,

First off, please forgive me if I'm posting this in the wrong section. I'm new to Java and I'm having a bit of trouble setting up and action event. I have an error with the following...

It's complaining about "previousButton.addActionListener(this);", saying that...

"non-static variable this cannot be referenced from a static context"

Can someone please explain to me what I'm doing wrong here?

Hi Steve
Try removing the keyword static from the declaration of your method, as follows:
public void addComponentsToPane(Container pane).
I am realtively new to java myself, I hope a more experienced person will post a reply to your question.

Hi samir! Thanks for replying!

If I remove the static from the addComponentsToPane method, I then get two errors.

1st problem: I now get the following error with the line that has the addActionListener...

"addActionListener(java.awt.event.ActionListener) in javax.swing.AbstractButton cannot be applied to (movietheater.MovieTheater)"

2nd problem: I get a bunch of errors like the following...

"non-static method addComponentsToPane(java.awt.Container) cannot be referenced from a static context"

If I remove static from every method, then that resolves my second problem, but is this a good solution? Also I don't understand my first problem.

I probably should have posted all of my code. Here it is...

package movietheater;
 
import java.awt.*;
import java.awt.event.ActionEvent;
import javax.swing.*;
 
public class MovieTheater {
 
final static boolean shouldFill = true;
final static boolean shouldWeightX = true;
final static boolean RIGHT_TO_LEFT = false;
 
/** Creates a new instance of MovieTheater */
public MovieTheater() {
}
 
/** Returns an ImageIcon, or null if the path was invalid. */
protected static ImageIcon createImageIcon(String path) {
java.net.URL imgURL = MovieTheater.class.getResource(path);
 
if (imgURL != null) {
return new ImageIcon(imgURL);
} else {
System.err.println("Couldn't find file: " + path);
return null;
}
}
 
public void addComponentsToPane(Container pane) {
if (RIGHT_TO_LEFT) {
pane.setComponentOrientation(ComponentOrientation.RIGHT_TO_LEFT);
}
 
JButton previousButton;
 
ImageIcon previousIcon = createImageIcon("images/LeftArrowGreen.png");
 
pane.setLayout(new GridBagLayout());
 
GridBagConstraints c = new GridBagConstraints();
 
if (shouldFill) {
// Natural height, maximum width.
c.fill = GridBagConstraints.HORIZONTAL;
}
 
if (shouldWeightX) {
c.weightx = 0.5;
}
 
// Create and place the Previous button.
previousButton = new JButton(previousIcon);
previousButton.setToolTipText("Previous Movie");
previousButton.setActionCommand("previous");
previousButton.addActionListener(this);
// c.ipadx = 15;
c.gridx = 0;
c.gridy = 1;
c.insets = new Insets(40, 20, 10, 20);// Padding: top, left, button, right.
pane.add(previousButton, c);
}
 
private void createAndShowGUI() {
// Make sure we have a nice window decorations.
JFrame.setDefaultLookAndFeelDecorated(true);
 
// Create and setup the window.
JFrame frame = new JFrame("Movie Theater");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
// Set up the conent pane.
addComponentsToPane(frame.getContentPane());
 
// Display the window.
frame.pack();
frame.setVisible(true);
}
 
    //User clicked either the next or the previous button.
    public void actionPerformed(ActionEvent e) {
        if (e.getActionCommand().equals("previous")) {
System.out.println("Previous!");
        }
    }
 
public void main(String[] args) {
try {
UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
} catch (Exception e) {
}
 
javax.swing.SwingUtilities.invokeLater(new Runnable () {
public void run() {
createAndShowGUI();
}
});
}
 
}

[ April 26, 2006: Message edited by: Steve Vittoria ]

Use static sparingly. "Real" programs use it very little. Removing the "static"s all over the place was a good thing.

As far as the other error goes: it's not enough to define the actionPerformed() method -- the class this appears in must also explain that it's an action listener by declaring that it implements the ActionListener interface; i.e.,

public class MyClass extends JWhatever implements ActionListener {

Thanks for the ActionListener tip, I think that solved that issue.

However if I remove static from all the methods, than I get an error stating "No main classes found". If I put back static in the main method, I then get the same error message as before...

"non-static method createAndShowGUI() cannot be referenced from a static context"

Hi Steve
The main method should always be static as far as I know.
It is better to create the GUI in the constructor, and take care of displaying it in the main.
Try this:

package movietheater;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class MovieTheater extends JPanel implements ActionListener {
 
final static boolean shouldFill = true;
final static boolean shouldWeightX = true;
final static boolean RIGHT_TO_LEFT = false;
 
/** Creates a new instance of MovieTheater */
public MovieTheater() {
if (RIGHT_TO_LEFT) {
setComponentOrientation(ComponentOrientation.RIGHT_TO_LEFT);
}
 
JButton previousButton;
 
ImageIcon previousIcon = createImageIcon("images/LeftArrowGreen.png");
 
setLayout(new GridBagLayout());
 
GridBagConstraints c = new GridBagConstraints();
 
if (shouldFill) {
// Natural height, maximum width.
c.fill = GridBagConstraints.HORIZONTAL;
}
 
if (shouldWeightX) {
c.weightx = 0.5;
}
 
// Create and place the Previous button.
previousButton = new JButton(previousIcon);
previousButton.setToolTipText("Previous Movie");
previousButton.setActionCommand("previous");
previousButton.addActionListener(this);
// c.ipadx = 15;
c.gridx = 0;
c.gridy = 1;
c.insets = new Insets(40, 20, 10, 20);// Padding: top, left, button, right.
add(previousButton, c);
}
 
/** Returns an ImageIcon, or null if the path was invalid. */
protected static ImageIcon createImageIcon(String path) {
java.net.URL imgURL = MovieTheater.class.getResource(path);
 
if (imgURL != null) {
return new ImageIcon(imgURL);
} else {
System.err.println("Couldn't find file: " + path);
return null;
}
}

//User clicked either the next or the previous button.
    public void actionPerformed(ActionEvent e) {
        if (e.getActionCommand().equals("previous")) {
System.out.println("Previous!");
        }
    }
 
public static void main(String[] args) {
try {
UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
} catch (Exception e) {
}
 
javax.swing.SwingUtilities.invokeLater(new Runnable () {
public void run() {
// Make sure we have a nice window decorations.
JFrame.setDefaultLookAndFeelDecorated(true);
 
// Create and setup the window.
JFrame frame = new JFrame("Movie Theater");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
// Set up the conent pane.
frame.getContentPane().add(new MovieTheater());
 
// Display the window.
frame.pack();
frame.setVisible(true);
}
});
}
 
}

Works perfectly now! Thanks samir and Ernest!

Now I'll have to take a closer look the code you provide and see what was wrong with me version.

Thanks again guys!

[ April 26, 2006: Message edited by: Steve Vittoria ]

You are welcome Steve.
Good luck!