Swing components - event driven programming - calculator

// Swing components
// calculator
// event driven programming 

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

class calculator implements ActionListener
{
     double n1, n2, result;
     JButton b1, b2, b3, b4, b5, b6, b7, b8, b9, b0;
     JButton add,sub,mul,div,eq,dot;
     JTextField tf;
     JFrame jf;
    
     JPanel p1, p2;
    
     String s1="";
     double Num1=0;
     double Num2=0;
     double res=0;
     String opn="";
    
     calculator()
     {
          jf = new JFrame();
          jf.setSize(250,200);
          jf.setTitle("Calculator");
         
         
          tf = new JTextField("0");
          tf.setEditable(false);
          tf.setHorizontalAlignment(JTextField.RIGHT);
          jf.add(tf, BorderLayout.NORTH);
         
          GridLayout GL = new GridLayout(4,4);
          p2 = new JPanel();
          p2.setLayout(GL);
         
          b1 = new JButton("1");
          b2 = new JButton("2");
          b3 = new JButton("3");
          b4 = new JButton("4");
          b5 = new JButton("5");
          b6 = new JButton("6");
          b7 = new JButton("7");
          b8 = new JButton("8");
          b9 = new JButton("9");
          b0 = new JButton("0");
          add = new JButton("+");
          sub = new JButton("-");
          mul = new JButton("*");
          div = new JButton("/");
          eq = new JButton("=");
          dot = new JButton(".");
             
          p2.add(b9);
          p2.add(b8);
          p2.add(b7);
          p2.add(div);
          p2.add(b6);
          p2.add(b5);
          p2.add(b4);
          p2.add(mul);
          p2.add(b3);
          p2.add(b2);
          p2.add(b1);
          p2.add(sub);
          p2.add(b0);
          p2.add(dot);
          p2.add(eq);
          p2.add(add);
         
          jf.add(p2, BorderLayout.CENTER);
         
          b0.addActionListener(this);
          b1.addActionListener(this);
          b2.addActionListener(this);
          b3.addActionListener(this);
          b4.addActionListener(this);
          b5.addActionListener(this);
          b6.addActionListener(this);
          b7.addActionListener(this);
          b8.addActionListener(this);
          b9.addActionListener(this);
          add.addActionListener(this);
          sub.addActionListener(this);
          mul.addActionListener(this);
          div.addActionListener(this);
          eq.addActionListener(this);
          dot.addActionListener(this);
         
          jf.setVisible(true);
          jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     }
    
     public void actionPerformed(ActionEvent ae)
     {
          String str = ae.getActionCommand();
         
          if((str.equals("+"))||(str.equals("-"))||(str.equals("*"))||(str.equals("/")))
          {
              Num1 = Double.parseDouble(s1);
              s1="";
              tf.setText("");
              opn=str;
          }
          else if(str.equals("="))
          {
              Num2 = Double.parseDouble(s1);
              s1="";
             
              char op = opn.charAt(0);
              switch(op)
              {
                   case '+':  res = Num1+Num2;
                                  break;
                   case '-':   res = Num1-Num2;
                                  break;
                   case '*':  res = Num1*Num2;
                                  break;
                   case '/':   res = Num1/Num2;
                                  break;                                
              }
             
              tf.setText(String.valueOf(res));
          }
          else
          {
              s1=s1+str;
              tf.setText(s1);
          }
     }
    
     public static void main(String as[])
     {
          new calculator();
     }
}

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