Listener combobox e textfield

Button jtxt = (Button) e.getSource();  
  JComboBox comboBox = (JComboBox) e.getSource();

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


public class ex3 extends JFrame{
    
    private final JPanel panel;
    private JLabel label = new JLabel("=");
      private JLabel resposta = new JLabel("    0");
    private JTextField txt1 = new JTextField("",15);
    private JTextField txt2 = new JTextField("",15);
    String[] opStrings = { "Soma", "Subtrai", "Multiplica", "Divide" };
    JComboBox opList = new JComboBox(opStrings);
    private Button calcular = new Button("Calcular");
    
    GridBagConstraints cons = new GridBagConstraints();
GridBagLayout layout = new GridBagLayout();
char op;
    boolean ok =false;
    ex3(){
        JFrame f = new JFrame("Editor de texto");
        f.setSize(600, 300);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        f.add(panel=new JPanel(layout), BorderLayout.CENTER);
        f.setVisible(true);
      
        cons.fill = GridBagConstraints.BOTH;
          
        cons.gridy = 0; cons.gridx = 0;
        panel.add(txt1, cons);
        cons.gridy = 0; cons.gridx = 1;
        panel.add(opList, cons);
        cons.gridy = 0; cons.gridx = 2;
        panel.add(txt2, cons);
        cons.gridy = 0; cons.gridx = 3;
        panel.add(label, cons);
        cons.gridy = 0; cons.gridx = 4;
        panel.add(resposta, cons);
        cons.gridwidth = 4; 
        cons.gridy = 1; cons.gridx = 0;
        panel.add(calcular, cons);
        pack();
        Listener l = new Listener();
        opList.addActionListener(l);
        calcular.addActionListener(l);
        
}
    class Listener implements ActionListener {
    	  public void actionPerformed(ActionEvent e) {
    		
    		  
    		  
    	   if (e.getSource() == calcular) {
    		   // Troca isso por um switch case
    		   if (opList.getSelectedIndex() == 0)    
               {  
               op = '+';   
               calcula();  
               }  
              else  
               if (opList.getSelectedIndex() == 1)  
               {op = '-';    calcula();  
               }  
               else  
               if (opList.getSelectedIndex() == 2)  
               {op = '*';    calcula();  
                     
               }  
               else  
               if (opList.getSelectedIndex() == 3)  
               {op = '/';    calcula();  
                
               }  
    		   
    		   
    		   
    	   }
    		  
    		  
    		  
    		  
    		  
    	    }
    	  
    	   } 
    	       
    	    
    	  
    	    public void calcula() {  
    	        
    	        try {  
    	              System.out.println("op ");
    	           Double v1 = new Double(txt1.getText());  
    	           Double v2 = new Double(txt2.getText());  
    	          
    	           double resultado;  
    	          
    	           switch (op) {  
    	             
    	              case '+': { resultado = v1.doubleValue() + v2.doubleValue(); break; }  
    	              case '-': { resultado = v1.doubleValue() - v2.doubleValue(); break; }  
    	              case '/': { resultado = v1.doubleValue() / v2.doubleValue(); break; }  
    	              case '*': { resultado = v1.doubleValue() * v2.doubleValue(); break; }  
    	              default:{ resultado = 0; break; }  
    	           }  
    	             
    	              resposta.setText(String.valueOf(resultado)); 
    	        }  
    	        catch (Exception exp) {  
    	           JOptionPane.showMessageDialog(null, "Verifique os números digitados.");  
    	        }  
    	       
    	  }  	  
    	  
  
    
    
    
   public static void main(String[] args)
    { 
        new ex3();
        
    }
}