Programming and Problem Solving with Java

14.2 H o w D o Y o u W r i t e a n Applet? | 665

within the event handler, the code is enclosed within a try-catch statement that handles nu-
meric input errors.

importjava.applet.Applet; // Applet class
importjava.awt.event.; // Event handling classes
importjava.awt.; // Uer interface classes

public classCalculator2 extendsApplet implementsActionListener
{
public voidactionPerformed(ActionEvent event)
//Handles events from the buttons in the applet
{
doublesecondOperand; // Holds input value
String whichButton; // Holds the button's name
// Get the operand, checking for numeric format error
try
{
secondOperand = Double.parseDouble(inputField.getText());
}
catch(NumberFormatException except)
{
secondOperand = 0.0; // If error, set to zero
}

whichButton = event.getActionCommand(); // Get the button's name

if (whichButton.equals("+")) // When the name is "+"

result = result + secondOperand; // add the operand

else if(whichButton.equals("-")) // When the name is "-"

result = result – secondOperand; // subtract operand

else

result = 0.0; // Clear result to zero

register.setText("" + result); // Display result

inputField.setText(""); // Clear input

}

Because we are extending Appletrather than JApplet, the J’s have all been removed from
class names in the following declarations.

private TextField inputField; // Data field

private Label register; // Result shown on screen

private doubleresult; // Keeps current value

public voidinit()

{

Label resultLabel; // Indicates output area