G.12.6. Using Interface Payable to Process Invoices and Employees Polymorphically

PayableInterfaceTest (Fig. G.27) illustrates that interface Payable can be used to process a set of Invoices and Employees polymorphically in a single application. Line 9 declares payableObjects and assigns it an array of four Payable variables. Lines 12–13 assign the references of Invoice objects to the first two elements of payableObjects. Lines 14–17 then assign the references of SalariedEmployee objects to the remaining two elements of payableObjects. These assignments are allowed because an Invoice is a Payable, a SalariedEmployee is an Employee and an Employee is a Payable. Lines 23–29 use the enhanced for statement to polymorphically process each Payable object in payableObjects, printing the object as a String, along with the payment amount due. Line 27 invokes method toString via a Payable interface reference, even though toString is not declared in interface Payable—all references (including those of interface types) refer to objects that extend Object and therefore have a toString method. (Method toString also can be invoked implicitly here.) Line 28 invokes Payable method getPaymentAmount to obtain the payment amount for each object in payableObjects, regardless of the actual type of the object. The output reveals that the method calls in lines 27–28 invoke the appropriate class’s implementation of methods toString and getPaymentAmount. For instance, when currentPayable refers to an Invoice during the first iteration of the for loop, class Invoice’s toString and getPaymentAmount execute.

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 1   // Fig. G.27: PayableInterfaceTest.java
 2   // Tests interface Payable.
 3
 4   public class PayableInterfaceTest
 5   {
 6      public static void main( String[] args )
 7      {
 8         // create four-element Payable array
 9         Payable[] payableObjects = new Payable[ 4 ];
10
11         // populate array with objects that implement Payable
12         payableObjects[ 0 ] = new Invoice( "01234", "seat", 2, 375.00 );
13         payableObjects[ 1 ] = new Invoice( "56789", "tire", 4, 79.95 );
14         payableObjects[ 2 ] =
15            new SalariedEmployee( "John", "Smith", "111-11-1111", 800.00 );
16         payableObjects[ 3 ] =
17            new SalariedEmployee( "Lisa", "Barnes", "888-88-8888", 1200.00 );
18
19         System.out.println(
20            "Invoices and Employees processed polymorphically: " );
21
22         // generically process each element in array payableObjects
23         for ( Payable currentPayable : payableObjects )
24         {
25            // output currentPayable and its appropriate payment amount
26            System.out.printf( "%s %s: $%,.2f ",
27               currentPayable.toString(),
28               "payment due", currentPayable.getPaymentAmount() );
29         } // end for
30      } // end main
31   } // end class PayableInterfaceTest

Fig. G.27 | Payable interface test program processing Invoices and Employees polymorphically.