[Chapter 5] 5.4 A Simple Calculator

5.4 A Simple Calculator

It is always helpful to see complete and somewhat useful examples after learning something new. Example 5.2 shows a working calculator that performs floating point addition, subtraction, multiplication, and division. Figure 5.4 shows the calculator in operation. The button in the lower left corner is a decimal point. This applet uses a number of classes that will be discussed later in the book (most notably, some layout managers and a Panel); try to ignore them for now. Focus on the action() and compute() methods; action() figures out which button was pressed, converting it to a digit (0-9 plus the decimal point) or an operator (=, +, -, *, /). As you build a number, it is displayed in the label lab, which conveniently serves to store the number in string form. The compute() method reads the label's text, converts it to a floating point number, does the computation, and displays the result in the label. The addButtons() method is a helper method to create a group of Button objects at one time.

Example 5.2: Calculator Source Code


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));

    }

}

Figure 5.4: Calculator applet

[Graphic: Figure 5-4]


Buttons		Canvas