Java Question

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I'm new to Java and I'm wonder if someone can help me with this.
I have a class that's constructor makes a Gui window, and it's main calls this constructor. The problem is the event handling doesn't work because the event handling code can't access the GUI window variables (It just gives errors). I tried putting the Gui code inside main,but then I get errors about accessing non static variables from inside the static main.
This is the code:

java Syntax (Toggle Plain Text)
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
 
public class SwingMenu implements ActionListener{
 
  public static void main(String[] args) {
 
SwingMenu s = new SwingMenu();
 
 
 
}
 
public SwingMenu(){
Toolkit toolkit = Toolkit.getDefaultToolkit();
 
    // Get size
    Dimension dimension = toolkit.getScreenSize();
    JFrame frame = new JFrame(" ");
    JPanel panel = new JPanel();
 
panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    JMenuBar menubar = new JMenuBar();
    JMenu filemenu = new JMenu("File");
    JMenuItem fileItem1 = new JMenuItem("Exit");
    filemenu.add(fileItem1);
    fileItem1.addActionListener(this);
    menubar.add(filemenu);
    frame.setJMenuBar(menubar);
    JTextArea textArea = new JTextArea(27,27);
    JScrollPane scrollPane = new JScrollPane(textArea);
    panel.add(scrollPane);
     JTextArea textArea2 = new JTextArea(3,3);
    JScrollPane scrollPane2 = new JScrollPane(textArea2);
    panel.add(scrollPane2);
frame.add(panel);
int height = dimension.height-30;
    frame.setSize(dimension.width,height);
    frame.setVisible(true);
}
 
 
public void actionPerformed(ActionEvent ev) {
 
  // this is where I want to handle the events
}
}
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class SwingMenu implements ActionListener{

  public static void main(String[] args) {

SwingMenu s = new SwingMenu();



}

public SwingMenu(){
Toolkit toolkit = Toolkit.getDefaultToolkit();

    // Get size
    Dimension dimension = toolkit.getScreenSize();
    JFrame frame = new JFrame(" ");
    JPanel panel = new JPanel();

panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    JMenuBar menubar = new JMenuBar();
    JMenu filemenu = new JMenu("File");
    JMenuItem fileItem1 = new JMenuItem("Exit");
    filemenu.add(fileItem1);
    fileItem1.addActionListener(this);
    menubar.add(filemenu);
    frame.setJMenuBar(menubar);
    JTextArea textArea = new JTextArea(27,27);
    JScrollPane scrollPane = new JScrollPane(textArea);
    panel.add(scrollPane);
     JTextArea textArea2 = new JTextArea(3,3);
    JScrollPane scrollPane2 = new JScrollPane(textArea2);
    panel.add(scrollPane2);
frame.add(panel);
int height = dimension.height-30;
    frame.setSize(dimension.width,height);
    frame.setVisible(true);
}


public void actionPerformed(ActionEvent ev) {

  // this is where I want to handle the events
}
}

Let me know if you need a better explanation.

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curt22

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since Sep 2007

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Dec 22nd, 2007

Re: Java Question

I think you might be better off listing your GUI components outside the methods. You still initialise your components inside the swingMenu method, but doing it this way gives global access within the class. For example,

java Syntax (Toggle Plain Text)
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
 
public class SwingMenu implements ActionListener{
   JMenuBar menuBar;
   JMenu filemenu;
   // etc
   public static void main(String[] args) {
      SwingMenu s = new SwingMenu();
   }
 
   public SwingMenu(){
      menuBar = new JMenuBar();
      filemenu = new JMenu("File");
      // etc
   }
}
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class SwingMenu implements ActionListener{
   JMenuBar menuBar;
   JMenu filemenu;
   // etc
   public static void main(String[] args) {
      SwingMenu s = new SwingMenu();
   }

   public SwingMenu(){
      menuBar = new JMenuBar();
      filemenu = new JMenu("File");
      // etc
   }
}

Then you can access the components within the actionPerformed method as needed.

Hope this helps.

darkagn

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since Aug 2007

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Dec 23rd, 2007

Re: Java Question

Hi curt22 ,

It's best practice to separate event handling method from the event source class(class which contains UI widgets). So that later it will be easy to change that without affecting event source class.

Consider this example :

java Syntax (Toggle Plain Text)
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
 
public class SwingMenu {
 
 
// Inner Class Implementation
 
 /* 
 
class actionListener implements ActionListener {
   public void actionPerformed(ActionEvent avt) {
   System.out.println("Triggered");
 }
}   */
 
public static void main(String[] args) {
 
    SwingMenu swingMenu = new SwingMenu();
 
 
    JMenuBar menuBar = new JMenuBar();
    JMenu file = new JMenu("File");
    JMenuItem demo = new JMenuItem("demo");
 
    //demo.addActionListener(swingMenu.new actionListener()); // Using Inner class Implementation 
 
    demo.addActionListener(new actionListener()); // Using a Separate class 
 
 
    file.add(demo);
    menuBar.add(file);
 
    JFrame frame = new JFrame("Frame");
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add(BorderLayout.NORTH,menuBar);
    frame.setSize(300,400);
  } 
}
 
// Event Handling class
 class actionListener implements ActionListener {
   public void actionPerformed(ActionEvent avt) {
   System.out.println("Triggered");
 }
}
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class SwingMenu {

 
// Inner Class Implementation

 /* 
 
class actionListener implements ActionListener {
   public void actionPerformed(ActionEvent avt) {
   System.out.println("Triggered");
 }
}   */

public static void main(String[] args) {
  
    SwingMenu swingMenu = new SwingMenu();
    

    JMenuBar menuBar = new JMenuBar();
    JMenu file = new JMenu("File");
    JMenuItem demo = new JMenuItem("demo");

    //demo.addActionListener(swingMenu.new actionListener()); // Using Inner class Implementation 
   
    demo.addActionListener(new actionListener()); // Using a Separate class 


    file.add(demo);
    menuBar.add(file);

    JFrame frame = new JFrame("Frame");
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add(BorderLayout.NORTH,menuBar);
    frame.setSize(300,400);
  } 
}

// Event Handling class
 class actionListener implements ActionListener {
   public void actionPerformed(ActionEvent avt) {
   System.out.println("Triggered");
 }
}

In this example I have written a separate class for event handling. you can use inner class or a separate class

Hope this helps

parthiban

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