جافا : GUI simple calc

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

import javax.swing.border.Border;

public class Ca extends JPanel implements ActionListener {

    double q ;

    double w ; 

    double e ;

JFrame a;

JPanel ma;

    JPanel but;

    JPanel win;

    JPanel in;

    JButton add;

    JButton min;

    JButton mul;

    JButton div;

    JTextField in1;

    JTextField in2;

    JTextField out;

    Ca(){

Border border = BorderFactory.createLineBorder(Color.GRAY);

a = new JFrame();

ma = new JPanel(new BorderLayout());

but = new JPanel( );

win = new JPanel(new GridLayout(2,1));

add=new JButton("add");

min=new JButton("min");

mul=new JButton("mul");

div=new JButton("div");

in1 = new JTextField();

in2 = new JTextField();

out = new JTextField();

in= new JPanel(new GridLayout(2,2));

JLabel s = new JLabel("first");

JLabel d = new JLabel("second");

in1.setHorizontalAlignment(SwingConstants.CENTER);

in2.setHorizontalAlignment(SwingConstants.CENTER);

out.setHorizontalAlignment(SwingConstants.CENTER);

out.setFont(new Font("serif",Font.BOLD,20));

s.setHorizontalAlignment(SwingConstants.CENTER);

d.setHorizontalAlignment(SwingConstants.CENTER);

      s.setBorder(border);

      d.setBorder(border);

add.addActionListener(this);

min.addActionListener(this);

mul.addActionListener(this);

div.addActionListener(this);

but.add(add);

but.add(min);

but.add(mul);

but.add(div);

but.setBackground(Color.BLUE);

        in.add(s);

in.add(d);

in.add(in1);

in.add(in2);

in.setBackground(Color.CYAN);

win.add(in);

win.add(out,FlowLayout.CENTER);

ma.add(but,BorderLayout.SOUTH);

ma.add(win,BorderLayout.NORTH);

   ma.setBackground(Color.BLACK);

a.add(ma);

        a.setTitle("Simple Calc");

a.setSize(400,150);

a.setLocationRelativeTo(a);

a.setResizable(false);

a.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

a.setVisible(true);

}

public static void main (String [ ] st){

new Ca();

}

public void actionPerformed(ActionEvent at) {

try{ q=Integer.parseInt(in1.getText());

w=Integer.parseInt(in2.getText());

if (at.getSource()==add)e = q+w;

else if (at.getSource()==min)e= q-w;

else if (at.getSource()==mul)e=q*w;

else if(at.getSource()==div)if(w!=0)e=q/w;else e=0;

out.setText(Double.toString(e));

}catch(Exception e){

JOptionPane.showMessageDialog(a, "Error","Error",0);

}

}}