Programming and Problem Solving with Java

CASE STUDY

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We still have one more step in mainto flesh out.

Registering an event listener for a button involves three steps: declare a variable of

the listener class, instantiate a listener object, and add the listener object to the button.

The listener for the numeric operations is, of course,NumericHandler.

For the clear operation, we just substitute ClearHandler.

We have now completed the responsibility algorithms for the application class

Calculator. The remaining classes handle the buttons. First, we design the listener for

the numeric buttons. We have said that the listener class is called NumericHandler. What

should the actionPerformedmethod do when the user clicks the add or subtract button?

It should get the value to be used in the calculation from the input field and convert it

to a real number. If the user enters a string that does not represent a legitimate real

number, a NumberFormatExceptionmay be thrown. In Chapter 9, we show you how to han-

dle this exception; here, we assume correct input.

Next, we determine which operation should be applied to the number, addition or

subtraction. Wait a minute: Addition and subtraction are binary operations! We have

one value; where is the other one? Actually, the value just input is the second operand.

The first operand is the value in the register. Rather than extract it from the window

each time, we should keep a copy in numeric form. (We must be sure to set that value

to zero originally.) Back to the question of determining which button was clicked: We

use the parameter in the actionPerformedmethod to tell us.

Register the Event Listener for the Clear Button

Declare a ClearHandler, clearOperation

Instantiate a ClearHandler for clearOperation

add clearOperation as listener for clear button

Register the Event Listener for the Arithmetic Buttons

Declare a NumericHandler, operation

Instantiate a NumericHandler for operation

add operation as listener for arithmetic buttons

Register Listeners with Buttons

Register the event listener for the arithmetic buttons

Register the event listener for the clear button

Add Each Interface Object to the Pane

Add resultLabel to calcPane

Add register to calcPane

Add entryLabel to calcPane

Add inputField to calcPane

Add add to calcPane

Add subtract to calcPane

Add clear to calcPane