import java.applet.Applet;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class AdderApplet extends Applet
    implements ActionListener
{

    public void init()
    {
        setLayout(new BorderLayout());
        Panel textPanel = new Panel();
        textPanel.setBackground(Color.gray);
        textPanel.setLayout(new FlowLayout());
        inputOutputField = new TextField("0", 20);
        inputOutputField.setBackground(Color.purple);
        textPanel.add(inputOutputField);
        add(textPanel, "North");
        Panel platformPanel = new Panel();
        platformPanel.setLayout(new GridLayout(4, 5));
        Panel button1Panel = new Panel();
        button1Panel.setBackground(Color.black);
        Button sevenButton = new Button(" 7 ");
        sevenButton.addActionListener(this);
        button1Panel.add(sevenButton);
        Button eightButton = new Button(" 8 ");
        eightButton.addActionListener(this);
        button1Panel.add(eightButton);
        Button nineButton = new Button(" 9 ");
        nineButton.addActionListener(this);
        button1Panel.add(nineButton);
        Button divideButton = new Button(" / ");
        divideButton.addActionListener(this);
        button1Panel.add(divideButton);
        platformPanel.add(button1Panel);
        Panel button2Panel = new Panel();
        button2Panel.setBackground(Color.black);
        Button fourButton = new Button(" 4 ");
        fourButton.addActionListener(this);
        button2Panel.add(fourButton);
        Button fiveButton = new Button(" 5 ");
        fiveButton.addActionListener(this);
        button2Panel.add(fiveButton);
        Button sixButton = new Button(" 6 ");
        sixButton.addActionListener(this);
        button2Panel.add(sixButton);
        Button multiplyButton = new Button(" * ");
        multiplyButton.addActionListener(this);
        button2Panel.add(multiplyButton);
        platformPanel.add(button2Panel);
        Panel button3Panel = new Panel();
        button3Panel.setBackground(Color.black);
        Button oneButton = new Button(" 1 ");
        oneButton.addActionListener(this);
        button3Panel.add(oneButton);
        Button twoButton = new Button(" 2 ");
        twoButton.addActionListener(this);
        button3Panel.add(twoButton);
        Button threeButton = new Button(" 3 ");
        threeButton.addActionListener(this);
        button3Panel.add(threeButton);
        Button minusButton = new Button(" - ");
        minusButton.addActionListener(this);
        button3Panel.add(minusButton);
        platformPanel.add(button3Panel);
        Panel button4Panel = new Panel();
        button4Panel.setBackground(Color.black);
        Button zeroButton = new Button(" 0 ");
        zeroButton.addActionListener(this);
        button4Panel.add(zeroButton);
        Button dotButton = new Button(" . ");
        dotButton.addActionListener(this);
        button4Panel.add(dotButton);
        Button addButton = new Button(" + ");
        addButton.addActionListener(this);
        button4Panel.add(addButton);
        Button equalsButton = new Button(" = ");
        equalsButton.addActionListener(this);
        button4Panel.add(equalsButton);
        platformPanel.add(button4Panel);
        add(platformPanel, "Center");
        Panel clearPanel = new Panel();
        clearPanel.setBackground(Color.blue);
        Button clearButton = new Button(" CLEAR ");
        clearButton.addActionListener(this);
        clearPanel.add(clearButton);
        add(clearPanel, "South");
    }

    public void actionPerformed(ActionEvent e)
    {
        if(e.getActionCommand().equals(" 7 "))
        {
            numString = numString + "7";
            inputOutputField.setText(numString);
            flag = "y";
        } else
        if(e.getActionCommand().equals(" 8 "))
        {
            numString = numString + "8";
            inputOutputField.setText(numString);
            flag = "y";
        } else
        if(e.getActionCommand().equals(" 9 "))
        {
            numString = numString + "9";
            inputOutputField.setText(numString);
            flag = "y";
        } else
        if(e.getActionCommand().equals(" 4 "))
        {
            numString = numString + "4";
            inputOutputField.setText(numString);
            flag = "y";
        } else
        if(e.getActionCommand().equals(" 5 "))
        {
            numString = numString + "5";
            inputOutputField.setText(numString);
            flag = "y";
        } else
        if(e.getActionCommand().equals(" 6 "))
        {
            numString = numString + "6";
            inputOutputField.setText(numString);
            flag = "y";
        } else
        if(e.getActionCommand().equals(" 1 "))
        {
            numString = numString + "1";
            inputOutputField.setText(numString);
            flag = "y";
        } else
        if(e.getActionCommand().equals(" 2 "))
        {
            numString = numString + "2";
            inputOutputField.setText(numString);
            flag = "y";
        } else
        if(e.getActionCommand().equals(" 3 "))
        {
            numString = numString + "3";
            inputOutputField.setText(numString);
            flag = "y";
        } else
        if(e.getActionCommand().equals(" - "))
        {
            equalBuffer = "";
            calcIt();
            buttonBuffer = "-";
        } else
        if(e.getActionCommand().equals(" / "))
        {
            equalBuffer = "";
            calcIt();
            buttonBuffer = "/";
        } else
        if(e.getActionCommand().equals(" * "))
        {
            equalBuffer = "";
            calcIt();
            buttonBuffer = "*";
        } else
        if(e.getActionCommand().equals(" 0 "))
        {
            numString = numString + "0";
            inputOutputField.setText(numString);
        } else
        if(e.getActionCommand().equals(" . "))
        {
            numString = numString + ".";
            inputOutputField.setText(numString);
        } else
        if(e.getActionCommand().equals(" + "))
        {
            equalBuffer = "";
            calcIt();
            buttonBuffer = "+";
        } else
        if(e.getActionCommand().equals(" = "))
        {
            calcIt();
            buttonBuffer = buttonBuffer;
            equalBuffer = "=";
        } else
        if(e.getActionCommand().equals(" CLEAR "))
        {
            numString = "";
            inputOutputField.setText("0");
            buttonBuffer = "";
            numBuffer = 0.0D;
            equalBuffer = "";
            equalString = "";
        } else
        {
            inputOutputField.setText("Error in adder code.");
        }
        repaint();
    }

    private void doTheMath()
    {
        if(buttonBuffer == "+")
        {
            if(numString == "")
                numString = "0";
            numBuffer = numBuffer + stringToDouble(numString);
            inputOutputField.setText(Double.toString(numBuffer));
        } else
        if(buttonBuffer == "-")
        {
            if(numString == "")
                numString = "0";
            numBuffer = numBuffer - stringToDouble(numString);
            inputOutputField.setText(Double.toString(numBuffer));
        } else
        if(buttonBuffer == "*")
        {
            if(numString == "")
                numString = "1";
            numBuffer = numBuffer * stringToDouble(numString);
            inputOutputField.setText(Double.toString(numBuffer));
        } else
        if(buttonBuffer == "/")
        {
            if(numString == "")
                numString = "1";
            numBuffer = numBuffer / stringToDouble(numString);
            inputOutputField.setText(Double.toString(numBuffer));
        }
    }

    private void doEqualMath()
    {
        if(buttonBuffer == "+")
        {
            numBuffer = numBuffer + stringToDouble(equalString);
            inputOutputField.setText(Double.toString(numBuffer));
        } else
        if(buttonBuffer == "-")
        {
            numBuffer = numBuffer - stringToDouble(equalString);
            inputOutputField.setText(Double.toString(numBuffer));
        } else
        if(buttonBuffer == "*")
        {
            numBuffer = numBuffer * stringToDouble(equalString);
            inputOutputField.setText(Double.toString(numBuffer));
        } else
        if(buttonBuffer == "/")
        {
            numBuffer = numBuffer / stringToDouble(equalString);
            inputOutputField.setText(Double.toString(numBuffer));
        }
    }

    private void calcIt()
    {
        numString = inputOutputField.getText();
        if(flag == "y")
        {
            equalString = inputOutputField.getText();
            doTheMath();
            numBuffer = stringToDouble(inputOutputField.getText());
            numString = "";
        }
        if(flag == "n")
        {
            if(equalBuffer == "=")
                doEqualMath();
            numBuffer = stringToDouble(inputOutputField.getText());
            if(equalBuffer != "=")
                buttonBuffer = "";
            numString = "";
        }
        flag = "n";
    }

    private static double stringToDouble(String stringObject)
    {
        return Double.valueOf(stringObject.trim()).doubleValue();
    }

    public AdderApplet()
    {
        buttonBuffer = "";
        equalBuffer = "";
        numBuffer = 0.0D;
        numString = "";
        equalString = "";
        flag = "n";
    }

    private TextField inputOutputField;
    private String buttonBuffer;
    private String equalBuffer;
    private double numBuffer;
    private String numString;
    private String equalString;
    private String flag;
}

<APPLET CODE="AdderApplet.class" WIDTH="175" HEIGHT="175">
</APPLET>