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Java AWT

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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;
            case '+':   savedValue += sValue;
            case '-':   savedValue -= sValue;
            case '*':   savedValue *= sValue;
            case '/':   savedValue /= sValue;
        lab.setText (String.valueOf(savedValue));

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