java calculator

 JAVA CALCULATOR

/**
  This is a simple calculator program can any one take and use.

   */
import javax.swing.*;
import javax.swing.JOptionPane;
import java.awt.*;
import java.awt.event.*;//<applet code=Calculator height=300 width=200></applet>
public class Calculator extends JApplet {
   public void init() {
      CalculatorPanel calc=new CalculatorPanel();
      getContentPane().add(calc);
      }
   }

   class CalculatorPanel extends JPanel implements ActionListener {
      JButton
n1,n2,n3,n4,n5,n6,n7,n8,n9,n0,plus,minus,mul,div,dot,equal;
      static JTextField result=new JTextField("0",45);
      static String lastCommand=null;
      JOptionPane p=new JOptionPane();
      double preRes=0,secVal=0,res;

      private static void assign(String no)
        {
         if((result.getText()).equals("0"))
            result.setText(no);
          else if(lastCommand=="=")
           {
            result.setText(no);
            lastCommand=null;
           }
          else
            result.setText(result.getText()+no);
         }

      public CalculatorPanel() {
         setLayout(new BorderLayout());
         result.setEditable(false);
         result.setSize(300,200);
         add(result,BorderLayout.NORTH);
         JPanel panel=new JPanel();
         panel.setLayout(new GridLayout(4,4));

         n7=new JButton("7");
         panel.add(n7);
         n7.addActionListener(this);
         n8=new JButton("8");
         panel.add(n8);
         n8.addActionListener(this);
         n9=new JButton("9");
         panel.add(n9);
         n9.addActionListener(this);
         div=new JButton("/");
         panel.add(div);
         div.addActionListener(this);

         n4=new JButton("4");
         panel.add(n4);
         n4.addActionListener(this);
         n5=new JButton("5");
         panel.add(n5);
         n5.addActionListener(this);
         n6=new JButton("6");
         panel.add(n6);
         n6.addActionListener(this);
         mul=new JButton("*");
         panel.add(mul);
         mul.addActionListener(this);

         n1=new JButton("1");
         panel.add(n1);
         n1.addActionListener(this);
         n2=new JButton("2");
         panel.add(n2);
         n2.addActionListener(this);
         n3=new JButton("3");
         panel.add(n3);
         n3.addActionListener(this);
         minus=new JButton("-");
         panel.add(minus);
         minus.addActionListener(this);

         dot=new JButton(".");
         panel.add(dot);
         dot.addActionListener(this);
         n0=new JButton("0");
         panel.add(n0);
         n0.addActionListener(this);
         
         equal=new JButton("=");
         panel.add(equal);
         equal.addActionListener(this);
         plus=new JButton("+");
         panel.add(plus);
         plus.addActionListener(this);
         add(panel,BorderLayout.CENTER);
      }
      public void actionPerformed(ActionEvent ae)
         {
      if(ae.getSource()==n1) assign("1");
      else if(ae.getSource()==n2) assign("2");
      else if(ae.getSource()==n3) assign("3");
      else if(ae.getSource()==n4) assign("4");
      else if(ae.getSource()==n5) assign("5");
      else if(ae.getSource()==n6) assign("6");
      else if(ae.getSource()==n7) assign("7");
      else if(ae.getSource()==n8) assign("8");
      else if(ae.getSource()==n9) assign("9");
      else if(ae.getSource()==n0) assign("0");
     
      else if(ae.getSource()==dot)
            {
             if(((result.getText()).indexOf("."))==-1)
                result.setText(result.getText()+".");
           }
      else if(ae.getSource()==minus)
             {
             preRes=Double.parseDouble(result.getText());
             lastCommand="-";
             result.setText("0");
             }
      else if(ae.getSource()==div)
             {
             preRes=Double.parseDouble(result.getText());
             lastCommand="/";
             result.setText("0");
             }
      else if(ae.getSource()==equal)
             {
             secVal=Double.parseDouble(result.getText());
             if(lastCommand.equals("/"))
                  res=preRes/secVal;
             else if(lastCommand.equals("*"))
                  res=preRes*secVal;
             else if(lastCommand.equals("-"))
                  res=preRes-secVal;
             else if(lastCommand.equals("+"))
                  res=preRes+secVal;
             result.setText(" "+res);
             lastCommand="=";
             }
      else if(ae.getSource()==mul)
             {
              preRes=Double.parseDouble(result.getText());
              lastCommand="*";
              result.setText("0");
              }
      else if(ae.getSource()==plus)
              {
              preRes=Double.parseDouble(result.getText());
              lastCommand="+";
              result.setText("0");
              }

       }
 }