Variaveis na calculadora

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class nada extends JFrame{


      public nada(){
      super("Meu Notepad");
      this.montaJanela();
    }

    private void montaJanela(){
	
		
			double total;
			JButton mais = new JButton("+");
			JButton menos = new JButton("-");
			JButton vezes = new JButton("*");
			JButton div = new JButton("/");
			JTextArea calc = new JTextArea(2,8);
			JTextArea calcu = new JTextArea(2,8);
			

	        GridBagLayout layout  = new GridBagLayout();
            GridBagConstraints c = new GridBagConstraints();
            getContentPane().setLayout(layout);

			c.gridy = 2; 
            c.gridx = 5; 
            layout.setConstraints(calc, c);
            getContentPane().add(calc);
			
			c.gridy = 6; 
            c.gridx = 8; 
            layout.setConstraints(calcu, c);
            getContentPane().add(calcu);
			
			c.gridy = 0; 
            c.gridx = 10; 
            layout.setConstraints(mais, c);
            getContentPane().add(mais);
			
			c.gridy = 10; 
            c.gridx = 10; 
            layout.setConstraints(menos, c);
            getContentPane().add(menos);
			
			c.gridy = 20; 
            c.gridx = 10; 
            layout.setConstraints(vezes, c);
            getContentPane().add(vezes);

			c.gridy = 30; 
            c.gridx = 10; 
            layout.setConstraints(div, c);
            getContentPane().add(div);
			div.addActionListener(new java.awt.event.ActionListener() {
			public void actionPerformed(java.awt.event.ActionEvent evt) {
			JOptionPane.showMessageDialog(null, "Total=");
     }
   });

	}

    public static void main(String[] args){
        //Cria objeto:
        nada janela = new nada();
		janela.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		janela.setVisible(true);
		janela.setSize(250,250);

    }
	}