Sci. Calc. Applet

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.lang.Math;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

/*
    <applet code="Calculator.class" width="400" height="400">
    </applet>
*/


/**
*Semi-Scientific Calculator [APPLET]
*@author Steve Robinson
*@version 1.1
*@see footyntech.wordpress.com
*/

public class Calculator extends JApplet {

        public void init()
        {
            //setTitle("My Scientific Calculator");
            EventQueue.invokeLater(new Runnable() {
                public void run()
                {
                    CalculatorPanel calcPanel=new CalculatorPanel();
                    add(calcPanel);
            //      pack();
                }
            });
        }
    }

    class CalculatorPanel extends JPanel
    {
        private JTextField display1;
        private JTextField display2;
        private JPanel buttonPanel;
        private JPanel displayPanel;

        ActionListener listener=new CalculatorListener();
        ActionListener displayListener=new DisplayListener();


        //Helper Methods to add buttons. Thanks to Core Java V1 book for this wonderful idea!

        void addDisplayButton(String type)
        {
            JButton button=new JButton(type);
            button.addActionListener(displayListener);
            buttonPanel.add(button);

        }

        void addCalcButton(String type)
        {
            JButton button=new JButton(type);
            button.addActionListener(listener);
            buttonPanel.add(button);

        }

        public CalculatorPanel()
        {


            //Layout for Main Calculator Panel
            setLayout(new BorderLayout());

            //Display Panel
            displayPanel=new JPanel();
            displayPanel.setLayout(new GridLayout(2,1));

            display1=new JTextField("");
            displayPanel.add(display1);

            display2=new JTextField("");
            displayPanel.add(display2);
            display1.requestFocusInWindow();

            add(displayPanel,BorderLayout.NORTH);

            //Panel for Buttons
            buttonPanel=new JPanel();
            //ButtonLayout
            buttonPanel.setLayout(new GridLayout(8,4));

            //Button Creation
            addCalcButton("e");

            addDisplayButton("DEL");
            addDisplayButton("AC");
            JButton dummyButton=new JButton("");
            buttonPanel.add(dummyButton);

            addCalcButton("log");
            addCalcButton("sin");
            addCalcButton("tan");
            addCalcButton("cos");
            addCalcButton("%");
            addCalcButton("x\u00B2");
            addCalcButton("x\u00B3");
            addCalcButton("sqrt");
            addCalcButton("1/x");
            addCalcButton("(");
            addCalcButton(")");
            addCalcButton("ANS");
            addCalcButton("7");
            addCalcButton("8");
            addCalcButton("9");
            addCalcButton("+");
            addCalcButton("4");
            addCalcButton("5");
            addCalcButton("6");
            addCalcButton("-");
            addCalcButton("1");
            addCalcButton("2");
            addCalcButton("3");
            addCalcButton("/");
            addCalcButton("0");
            addCalcButton(".");
            addCalcButton("=");
            addCalcButton("*");


            add(buttonPanel,BorderLayout.CENTER);


        }

        ScriptEngineManager manager=new ScriptEngineManager();
        ScriptEngine engine=manager.getEngineByName("JavaScript");
        boolean opDone=false;

        private class DisplayListener implements ActionListener
        {
            public void actionPerformed(ActionEvent event)
            {
                String command=event.getActionCommand();

                if(command.equals("AC"))
                {
                    display1.setText("");
                    display2.setText("");
                }
                else if(command.equals("OFF"))
                {
                    System.exit(0);
                }
                else if(command.equals("DEL"))
                {
                    display1.setText(display1.getText().substring(0, display1.getText().length()-1));
                }
            }
        }

        private class CalculatorListener implements ActionListener
        {
            public void actionPerformed (ActionEvent event)
            {
                String command=event.getActionCommand();

                try{
                    if(command.equals("log"))
                    {
                        double input=evaluateExpression(display1.getText());
                        display2.setText(Double.toString(Math.log(input)));
                        opDone=true;
                    }
                    else if(command.equals("sin"))
                    {
                        double input=evaluateExpression(display1.getText());
                        display2.setText(Double.toString(Math.sin(input)));
                        opDone=true;
                    }
                    else if(command.equals("tan"))
                    {
                        double input=evaluateExpression(display1.getText());
                        display2.setText(Double.toString(Math.tan(input)));
                        opDone=true;
                    }
                    else if(command.equals("cos"))
                    {
                        double input=evaluateExpression(display1.getText());
                        display2.setText(Double.toString(Math.cos(input)));
                        opDone=true;
                    }
                    else if(command.equals("e"))
                    {
                        double input=evaluateExpression(display1.getText());
                        display2.setText(Double.toString(Math.exp(input)));
                        opDone=true;
                    }
                    else if(command.equals("x\u00B2"))
                    {
                        double input=evaluateExpression(display1.getText());
                        display2.setText(Double.toString(input*input));
                        opDone=true;
                    }
                    else if(command.equals("x\u00B3"))
                    {
                        double input=evaluateExpression(display1.getText());
                        display2.setText(Double.toString(input*input*input));
                        opDone=true;
                    }
                    else if(command.equals("sqrt"))
                    {
                        double input=evaluateExpression(display1.getText());
                        display2.setText(Double.toString(Math.sqrt(input)));
                        opDone=true;
                    }
                    else if(command.equals("="))
                    {
                        String input=display1.getText();
                        //System.out.println(input);
                        display2.setText(Double.toString(evaluateExpression(input)));
                        opDone=true;
                    }
                    else if(command.equals("ANS"))
                    {
                        if(opDone)
                        {
                                display1.setText(display2.getText());
                        }
                        else
                        {
                                display1.setText(display1.getText()+display2.getText());
                        }

                        opDone=false;

                    }
                    else if(command.equals("1/x"))
                    {
                        double input=evaluateExpression(display1.getText());
                        display2.setText(Double.toString(1/input));
                        opDone=true;
                    }
                    else
                    {
                        char opr=command.charAt(0);
                        if(opDone)
                        {
                            if(Character.isDigit(opr)||opr=='.')
                            {
                                display1.setText(command);
                            }
                            else
                                display1.setText(display2.getText()+command);
                        }
                        else
                            display1.setText(display1.getText()+command);
                        opDone=false;
                    }
                }
                catch (Exception ex)
                {
                    ex.printStackTrace();
                }


            }

            Double evaluateExpression (String expression) throws Exception
            {
                //System.out.println(expression);
                return (Double) engine.eval(expression);
            }
        }



}