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Implement the Currency Converter Classes

The Converter class needs a method to perform the conversion computation. To add this method to Converter.java:

1. In the Groups & Files list in the project window, select the Classes group.

2. In the detail view, double-click Converter.java. Xcode opens the file in an editor window.

3. Insert the tagged lines in Listing 2-1 into the Converter.java file.

Listing 2-1  Implementation of the Converter class

/* Converter */

import com.apple.cocoa.foundation.*;

import com.apple.cocoa.application.*;

public class Converter {

    public float convertDollars(float dollars, float rate) {       // 1

        return dollars * rate;                                     // 2

    }                                                              // 3

}

The convertDollars method multiplies the two arguments and returns the result.

Now, update the empty implementation of the convert method in ConverterController.java that Interface Builder generated for you by changing or adding the tagged lines in Listing 2-2 to the file.

Listing 2-2  Implementation of the ConverterController class

import com.apple.cocoa.foundation.*;

import com.apple.cocoa.application.*;

public class ConverterController {

    public NSTextField amountField; /* IBOutlet */

    public Converter converter; /* IBOutlet */                     // 1

    public NSTextField dollarField; /* IBOutlet */

    public NSTextField rateField; /* IBOutlet */

    public void convert(Object sender) { /* IBAction */

        float dollars, rate, amount;                               // 2

        dollars = dollarField.floatValue();                        // 3

        rate = rateField.floatValue();                             // 4

        amount = converter.convertDollars(dollars, rate);          // 5

        amountField.setFloatValue(amount);                         // 6

        rateField.selectText(this);                                // 7

    }

}

Notice that line 1 changes the type of the converter instance variable from Object to Converter. This is possible here because the Converter class is now defined in the project (when the ConverterController class was defined in Interface Builder, the Converter class didn’t exist). You could have also done this by explicitly setting the type of the converter outlet to Converter within Interface Builder after defining the Converter class. This is an important step because leaving converter’s type as Object prevents the program from accessing the methods that the Converter class implements. (The Objective-C language doesn’t have this limitation.)

The convert method does the following:

• Gets the floating-point values typed into the Exchange Rate and Dollars to Convert text fields (lines 3 and 4).

• Invokes the convertDollars method and stores the result (line 5).

• Uses setFloatValue to write the returned value in the Amount in Other Currency text field (line (line 6).

• Invokes selectText on the rate field (line 7). This selects any text in the text field or, if there is no text, places the insertion point so the user can enter text into the text field.

You’ve now completed the implementation of Currency Converter. Are you surprised how little code you had to write, given that your application now has a fully functional converting system and a beautiful Aqua user interface? “Building Currency Converter” shows how to build and run the application.

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