Hello!
I'm currently working on a sort of simple imperial-to-metric converter application for myself, but I seem to have hit a snag. I've got the UI built, and the logic for one of the calculations built, but the button to make it all happen is giving me a problem.

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;
import java.awt.*;
import java.awt.event.*;
	 
	public class MetricConverterApp extends JFrame implements ActionListener
	{
		public MetricConverterApp() 
	    {
			
			double kilogram = 0.45359;
			
	         //Title section
	        setTitle("Metric Converter"); 
	        setSize(300,150);
	        setVisible(true);
	        setResizable(false);
	        
	        JTabbedPane jtp = new JTabbedPane();
	        
	        //Builds template of the window
	       getContentPane().add(jtp);
	 
	     //creating an instance of the menu bar for across the program
			MenuBar mnuBar = new MenuBar();
			setMenuBar(mnuBar);
			
			//building and filling the program's File menu
			Menu mnuFile = new Menu("File", true);
			mnuBar.add(mnuFile);
				MenuItem mnuFileExit = new MenuItem("Exit");
				mnuFile.add(mnuFileExit);
				
				
			//building and filling the program's About menu
			Menu mnuAbout = new Menu("About", true);
			mnuBar.add(mnuAbout);
				MenuItem mnuAboutMetricConverter = new MenuItem("About Metric Converter");
				mnuAbout.add(mnuAboutMetricConverter);
				
			//adding ActionListener to each of the menu items
			mnuFileExit.addActionListener(this);
			mnuAboutMetricConverter.addActionListener(this);
			
			//give each menu item an ActionCommand
			mnuFileExit.setActionCommand("Exit");
			mnuAboutMetricConverter.setActionCommand("About");
			
			
	       JPanel jp1 = new JPanel();//Builds first tab
	       jp1.setLayout(new BorderLayout());//Sets up layout of first tab
	       JPanel jp2 = new JPanel();//Builds second tab
	       jp2.setLayout(new BorderLayout());//Sets up layout of second tab
	       JPanel jp3 = new JPanel();//Builds third tab
	       jp3.setLayout(new BorderLayout());//sets up layout of second tab
	       

	       //Builds prompt for first tab
	       JLabel weightPrompt = new JLabel();
	       weightPrompt.setText("Enter the weight in pounds");
	       jp1.add(weightPrompt, BorderLayout.NORTH);
	       
	       //Builds text input for first tab
	       JTextField weightInput = new JTextField();
	       jp1.add(weightInput, BorderLayout.CENTER);
	       
	       //Builds button for first tab
	       JButton metWeight = new JButton("Convert to kilograms");
	       jp1.add(metWeight, BorderLayout.EAST);
	       metWeight.addActionListener(this);
	       
	       
	       //Builds text output for first tab
	       JTextField weightOutput = new JTextField();
	       jp1.add(weightOutput, BorderLayout.SOUTH);
	       
	       
	       //Builds prompt for second tab
	       JLabel lengthPrompt = new JLabel();
	       lengthPrompt.setText("Enter the length in feet");
	       jp2.add(lengthPrompt, BorderLayout.NORTH);
	       
	       //Builds text input for first tab
	       JTextField lengthInput = new JTextField("");
	       jp2.add(lengthInput, BorderLayout.CENTER);
	       
	       //Builds button for second tab
	       JButton metLength = new JButton("Convert to meters");
	       jp2.add(metLength, BorderLayout.EAST);
	       
	       //Builds text output for second tab
	       JTextField lengthOutput = new JTextField();
	       jp2.add(lengthOutput, BorderLayout.SOUTH);
	       
	       
	       //Build prompt for third tab
	       JLabel tempPrompt = new JLabel();
	       tempPrompt.setText("Enter the temperature in farenheit");
	       jp3.add(tempPrompt, BorderLayout.NORTH);
	       
	       //Builds text input for third tab
	       JTextField tempInput = new JTextField("");
	       jp3.add(tempInput, BorderLayout.CENTER);
	       
	       //Builds button for third tab
	       JButton metTemp = new JButton("Convert to celcius");
	       jp3.add(metTemp, BorderLayout.EAST);
	       
	       //Builds text output for third tab
	       JTextField tempOutput = new JTextField();
	       jp3.add(tempOutput, BorderLayout.SOUTH);
	 
	       //Adds tabs to pane
	       jtp.addTab("Weight", jp1);
	       jtp.addTab("Length", jp2);
	       jtp.addTab("Temperature", jp3);
	    }
	     
		@Override
		public void actionPerformed(ActionEvent e) 
		{
			//ActionListener for menu item clicks
       		String arg = e.getActionCommand();
       		if (arg == "Exit")
       			System.exit(0);
       		
       		if (arg == "About")
       		{
       			String message = "Metric Converter v1.0\nDesigned by:\nMe\nFebruary 19, 2011";
       			JOptionPane.showMessageDialog(null,message,"About Metric Converter", JOptionPane.INFORMATION_MESSAGE);
       		}
       		
       		if(e.getSource() == metWeight)
       		{
       		    double value = Double.parseDouble(weightInput.getText());
           		weightOutput.setText((value * kilogram) + "");
       		}
		}

		//main method
	     public static void main (String []args)
	     {
	        MetricConverterApp tab = new MetricConverterApp();
	        
	    }
}

The problem lies in lines 137 - 141. the metWeight and the kilogram variables are kicking back the error of "Cannot be resolved to a variable", and then the weightInput and weightOutput are showing an error of just "Cannot be resolved". When I run the program, I enter in the weight in pounds, then click the button to calculate it to kilograms, but nothing happens, and I think it's somewhere in that bit of code. I was wondering if anyone had any idea what could be causing this? I'd really appreciate any help.

welcome on this forum,

public class MetricConverterApp extends JFrame implements ActionListener{

JButton metWeight;

public MetricConverterApp(){

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metWeight = new JButton("Convert to kilograms");
jp1.add(metWeight, BorderLayout.EAST);
metWeight.addActionListener(this);

and every JTextField (where you want to input Numeric value) should be JFormattedTextField

don't extends and implements in main Class

Ok, I added the metWeight, weightInput, weightOutput, and kilogram between the class and the constructor like so:

public class MetricConverterApp extends JFrame implements ActionListener
	{
		JButton metweight;
		JFormattedTextField weightInput;
		JTextField weightOutput;
		double kilogram = 0.45359;
		
		
		public MetricConverterApp() 
	    {

However, the metWeight is still giving me problems...the same error is present, and I've looked at several examples and I've got it right...it's just not working.