// This example is from the book _Java AWT Reference_ by John Zukowski.
// Written by John Zukowski.  Copyright (c) 1997 O'Reilly & Associates.
// You may study, use, modify, and distribute this example for any purpose.
// This example is provided WITHOUT WARRANTY either expressed or
import java.awt.*;
import java.applet.*;

public class JavaCalc extends Applet {
    Label lab;
    boolean firstDigit = true;
    float savedValue = 0.0f;     // Initial value
    String operator = "=";  // Initial operator
    public void addButtons (Panel p, String labels) {
        int count = labels.length();
        for (int i=0;i<count;i++)
            p.add (new Button (labels.substring(i,i+1)));
    }
    public void init () {
        setLayout (new BorderLayout());
        add ("North", lab = new Label ("0", Label.RIGHT));
        Panel p = new Panel();
        p.setLayout (new GridLayout (4, 4));
        addButtons (p, "789/");
        addButtons (p, "456*");
        addButtons (p, "123-");
        addButtons (p, ".0=+");
        add ("Center", p);
    }
    public boolean action (Event e, Object o) {
        if (e.target instanceof Button) {
            String s = (String)o;
            if ("0123456789.".indexOf (s) != -1) {  // isDigit
                if (firstDigit) {
                    firstDigit = false;
                    lab.setText (s);
                } else {
                    lab.setText (lab.getText() + s);
                }
            } else {  // isOperator
                if (!firstDigit) {
                    compute (lab.getText());
                    firstDigit = true;
                }
                operator = s;
            }
            return true;
        }
        return false;
    }
    public void compute (String s) {
        float sValue = new Float (s).floatValue();
        char c = operator.charAt (0);
        switch (c) {
            case '=':   savedValue  = sValue;
                        break;
            case '+':   savedValue += sValue;
                        break;
            case '-':   savedValue -= sValue;
                        break;
            case '*':   savedValue *= sValue;
                        break;
            case '/':   savedValue /= sValue;
                        break;
        }
        lab.setText (String.valueOf(savedValue));
    }
}