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10.7 Case Study: Creating and Using Interfaces

Our next example (Figs. 10.11–10.15) reexamines the payroll system of Section 10.5. Suppose that the company involved wishes to perform several accounting operations in a single accounts payable application—in addition to calculating the earnings that must be paid to each employee, the company must also calculate the payment due on each of several invoices (i.e., bills for goods purchased). Though applied to unrelated things (i.e., employees and invoices), both operations have to do with obtaining some kind of payment amount. For an employee, the payment refers to the employee's earnings. For an invoice, the payment refers to the total cost of the goods listed on the invoice. Can we calculate such different things as the payments due for employees and invoices in a single application polymorphically? Does Java offer a capability requiring that unrelated classes implement a set of common methods (e.g., a method that calculates a payment amount)? Java interfaces offer exactly this capability.

Standardizing Interactions

Interfaces define and standardize the ways in which things such as people and systems can interact with one another. For example, the controls on a radio serve as an interface between radio users and a radio's internal components. The controls allow users to perform only a limited set of operations (e.g., change the station, adjust the volume, choose between AM and FM), and different radios may implement the controls in different ways (e.g., using push buttons, dials, voice commands). The interface specifies what operations a radio must permit users to perform but does not specify how the operations are performed.

Software Objects Communicate Via Interfaces

Software objects also communicate via interfaces. A Java interface describes a set of methods that can be called on an object to tell it, for example, to perform some task or return some piece of information. The next example introduces an interface named Payable to describe the functionality of any object that must be capable of being paid and thus must offer a method to determine the proper payment amount due. An interface declaration begins with the keyword interface and contains only constants and abstract methods. Unlike classes, all interface members must be public, and interfaces may not specify any implementation details, such as concrete method declarations and instance variables. All methods declared in an interface are implicitly public abstract methods, and all fields are implicitly public, static and final. [Note: As of Java SE 5, it became a better programming practice to declare sets of constants as enumerations with keyword enum. See Section 6.9 for an introduction to enum and Section 8.9 for additional enum details.]

Using an Interface

To use an interface, a concrete class must specify that it implements the interface and must declare each method in the interface with the signature specified in the interface declaration. To specify that a class implements an interface add the implements keyword and the name of the interface to the end of your class declaration's first line. A class that does not implement all the methods of the interface is an abstract class and must be declared abstract. Implementing an interface is like signing a contract with the compiler that states, "I will declare all the methods specified by the interface or I will declare my class abstract."

Relating Disparate Types

An interface is often used when disparate (i.e., unrelated) classes need to share common methods and constants. This allows objects of unrelated classes to be processed polymorphically—objects of classes that implement the same interface can respond to the same method calls. You can create an interface that describes the desired functionality, then implement this interface in any classes that require that functionality. For example, in the accounts payable application developed in this section, we implement interface Payable in any class that must be able to calculate a payment amount (e.g., Employee, Invoice).

Interfaces vs. Abstract Classes

An interface is often used in place of an abstract class when there's no default implementation to inherit—that is, no fields and no default method implementations. Like public abstract classes, interfaces are typically public types. Like a public class, a public interface must be declared in a file with the same name as the interface and the .java file-name extension.

Tagging Interfaces

We'll see in Chapter 17, Files, Streams and Object Serialization, the notion of "tagging interfaces"—empty interfaces that have no methods or constant values. They're used to add is-a relationships to classes. For example, in Chapter 17 we'll discuss a mechanism called object serialization, which can convert objects to byte representations and can convert those byte representations back to objects. To enable this mechanism to work with your objects, you simply have to mark them as Serializable by adding implements Serializable to the end of your class declaration's first line. Then, all the objects of your class have the is-a relationship with Serializable.

10.7.1 Developing a Payable Hierarchy

To build an application that can determine payments for employees and invoices alike, we first create interface Payable, which contains method getPaymentAmount that returns a double amount that must be paid for an object of any class that implements the interface. Method getPaymentAmount is a general-purpose version of method earnings of the Employee hierarchy—method earnings calculates a payment amount specifically for an Employee, while getPaymentAmount can be applied to a broad range of unrelated objects. After declaring interface Payable, we introduce class Invoice, which implements interface Payable. We then modify class Employee such that it also implements interface Payable. Finally, we update Employee subclass SalariedEmployee to "fit" into the Payable hierarchy by renaming SalariedEmployee method earnings as getPaymentAmount.

Classes Invoice and Employee both represent things for which the company must be able to calculate a payment amount. Both classes implement the Payable interface, so a program can invoke method getPaymentAmount on Invoice objects and Employee objects alike. As we'll soon see, this enables the polymorphic processing of Invoices and Employees required for the company's accounts payable application.

The UML class diagram in Fig. 10.10 shows the hierarchy used in our accounts payable application. The hierarchy begins with interface Payable. The UML distinguishes an interface from other classes by placing the word "interface" in guillemets (" and ") above the interface name. The UML expresses the relationship between a class and an interface through a relationship known as realization. A class is said to "realize," or implement, the methods of an interface. A class diagram models a realization as a dashed arrow with a hollow arrowhead pointing from the implementing class to the interface. The diagram in Fig. 10.10 indicates that classes Invoice and Employee each realize (i.e., implement) interface Payable. As in the class diagram of Fig. 10.2, class Employee appears in italics, indicating that it's an abstract class. Concrete class SalariedEmployee extends Employee and inherits its superclass's realization relationship with interface Payable.

Fig. 10.10

Fig. 10.10 Payable interface hierarchy UML class diagram.

10.7.2 Interface Payable

The declaration of interface Payable begins in Fig. 10.11 at line 4. Interface Payable contains public abstract method getPaymentAmount (line 6). The method is not explicitly declared public or abstract. Interface methods are always public and abstract, so they do not need to be declared as such. Interface Payable has only one method—interfaces can have any number of methods. In addition, method getPaymentAmount has no parameters, but interface methods can have parameters. Interfaces may also contain fields that are implicitly final and static.

Fig 10.11. Payable interface declaration.

1   // Fig. 10.11: Payable.java
2   // Payable interface declaration.
4   public interface Payable                                             
5   {                                                                    
6   double getPaymentAmount(); // calculate payment; no implementation
7   } // end interface Payable

10.7.3 Class Invoice

We now create class Invoice (Fig. 10.12) to represent a simple invoice that contains billing information for only one kind of part. The class declares private instance variables partNumber, partDescription, quantity and pricePerItem (in lines 6–9) that indicate the part number, a description of the part, the quantity of the part ordered and the price per item. Class Invoice also contains a constructor (lines 12–19), get and set methods (lines 22–74) that manipulate the class's instance variables and a toString method (lines 77–83) that returns a String representation of an Invoice object. Methods setQuantity (lines 46–52) and setPricePerItem (lines 61–68) ensure that quantity and pricePerItem obtain only nonnegative values.

Fig 10.12. Invoice class that implements Payable.

 1   // Fig. 10.12: Invoice.java
 2   // Invoice class that implements Payable.
 4   public class Invoice implements Payable
 5   {
 6      private String partNumber;
 7      private String partDescription;
 8      private int quantity;
 9      private double pricePerItem;
11      // four-argument constructor
12      public Invoice( String part, String description, int count,
13         double price )
14      {
15         partNumber = part;
16         partDescription = description;
17         setQuantity( count ); // validate and store quantity
18         setPricePerItem( price ); // validate and store price per item
19      } // end four-argument Invoice constructor
21      // set part number
22      public void setPartNumber( String part )
23      {
24         partNumber = part; // should validate
25      } // end method setPartNumber
27      // get part number
28      public String getPartNumber()
29      {
30         return partNumber;
31      } // end method getPartNumber
33      // set description
34      public void setPartDescription( String description )
35      {
36         partDescription = description; // should validate
37      } // end method setPartDescription
39      // get description
40      public String getPartDescription()
41      {
42         return partDescription;
43      } // end method getPartDescription
45      // set quantity
46      public void setQuantity( int count )
47      {
48         if ( count >= 0 )
49            quantity = count;
50         else
51            throw new IllegalArgumentException( "Quantity must be >= 0" );
52      } // end method setQuantity
54      // get quantity
55      public int getQuantity()
56      {
57         return quantity;
58      } // end method getQuantity
60      // set price per item
61      public void setPricePerItem( double price )
62      {
63         if ( price >= 0.0 )
64            pricePerItem = price;
65         else
66            throw new IllegalArgumentException(
67               "Price per item must be >= 0" );
68      } // end method setPricePerItem
70      // get price per item
71      public double getPricePerItem()
72      {
73         return pricePerItem;
74      } // end method getPricePerItem
76      // return String representation of Invoice object
77      @Override
78      public String toString()
79      {
80         return String.format( "%s: \n%s: %s (%s) \n%s: %d \n%s: $%,.2f",
81            "invoice", "part number", getPartNumber(), getPartDescription(),
82            "quantity", getQuantity(), "price per item", getPricePerItem() );
83      } // end method toString
85      // method required to carry out contract with interface Payable     
86      @Override                                                           
87      public double getPaymentAmount()                                    
88      {                                                                   
89      return getQuantity() * getPricePerItem(); // calculate total cost
90      } // end method getPaymentAmount                                    
91   } // end class Invoice

Line 4 indicates that class Invoice implements interface Payable. Like all classes, class Invoice also implicitly extends Object. Java does not allow subclasses to inherit from more than one superclass, but it allows a class to inherit from one superclass and implement as many interfaces as it needs. To implement more than one interface, use a comma-separated list of interface names after keyword implements in the class declaration, as in:

public class ClassName extends SuperclassName implements FirstInterface, SecondInterface, ... 

Class Invoice implements the one method in interface Payable—method getPaymentAmount is declared in lines 86–90. The method calculates the total payment required to pay the invoice. The method multiplies the values of quantity and pricePerItem (obtained through the appropriate get methods) and returns the result (line 89). This method satisfies the implementation requirement for this method in interface Payable—we've fulfilled the interface contract with the compiler.

10.7.4 Modifying Class Employee to Implement Interface Payable

We now modify class Employee such that it implements interface Payable. Figure 10.13 contains the modified class, which is identical to that of Fig. 10.4 with two exceptions. First, line 4 of Fig. 10.13 indicates that class Employee now implements interface Payable. So we must rename earnings to getPaymentAmount throughout the Employee hierarchy. As with method earnings in the version of class Employee in Fig. 10.4, however, it does not make sense to implement method getPaymentAmount in class Employee because we cannot calculate the earnings payment owed to a general Employee—we must first know the specific type of Employee. In Fig. 10.4, we declared method earnings as abstract for this reason, so class Employee had to be declared abstract. This forced each Employee concrete subclass to override earnings with an implementation.

Fig 10.13. Employee class that implements Payable.

 1   // Fig. 10.13: Employee.java
 2   // Employee abstract superclass that implements Payable.
 4   public abstract class Employee implements Payable
 5   {
 6      private String firstName;
 7      private String lastName;
 8      private String socialSecurityNumber;
10      // three-argument constructor
11      public Employee( String first, String last, String ssn )
12      {
13         firstName = first;
14         lastName = last;
15         socialSecurityNumber = ssn;
16      } // end three-argument Employee constructor
18      // set first name
19      public void setFirstName( String first )
20      {
21         firstName = first; // should validate
22      } // end method setFirstName
24      // return first name
25      public String getFirstName()
26      {
27         return firstName;
28      } // end method getFirstName
30      // set last name
31      public void setLastName( String last )
32      {
33         lastName = last; // should validate
34      } // end method setLastName
36      // return last name
37      public String getLastName()
38      {
39         return lastName;
40      } // end method getLastName
42      // set social security number
43      public void setSocialSecurityNumber( String ssn )
44      {
45         socialSecurityNumber = ssn; // should validate
46      } // end method setSocialSecurityNumber
48      // return social security number
49      public String getSocialSecurityNumber()
50      {
51         return socialSecurityNumber;
52      } // end method getSocialSecurityNumber
54      // return String representation of Employee object
55      @Override
56      public String toString()
57      {
58         return String.format( "%s %s\nsocial security number: %s",
59            getFirstName(), getLastName(), getSocialSecurityNumber() );
60      } // end method toString
62      // Note: We do not implement Payable method getPaymentAmount here so 
63      // this class must be declared abstract to avoid a compilation error.
64   } // end abstract class Employee

In Fig. 10.13, we handle this situation differently. Recall that when a class implements an interface, it makes a contract with the compiler stating either that the class will implement each of the methods in the interface or that the class will be declared abstract. If the latter option is chosen, we do not need to declare the interface methods as abstract in the abstract class—they're already implicitly declared as such in the interface. Any concrete subclass of the abstract class must implement the interface methods to fulfill the superclass's contract with the compiler. If the subclass does not do so, it too must be declared abstract. As indicated by the comments in lines 62–63, class Employee of Fig. 10.13 does not implement method getPaymentAmount, so the class is declared abstract. Each direct Employee subclass inherits the superclass's contract to implement method getPaymentAmount and thus must implement this method to become a concrete class for which objects can be instantiated. A class that extends one of Employee's concrete subclasses will inherit an implementation of getPaymentAmount and thus will also be a concrete class.

10.7.5 Modifying Class SalariedEmployee for Use in the Payable Hierarchy

Figure 10.14 contains a modified SalariedEmployee class that extends Employee and fulfills superclass Employee's contract to implement Payable method getPaymentAmount. This version of SalariedEmployee is identical to that of Fig. 10.5, but it replaces method earnings with method getPaymentAmount (lines 34–38). Recall that the Payable version of the method has a more general name to be applicable to possibly disparate classes. The remaining Employee subclasses (e.g., HourlyEmployee, CommissionEmployee and Base-PlusCommissionEmployee) also must be modified to contain method getPaymentAmount in place of earnings to reflect the fact that Employee now implements Payable. We leave these modifications as an exercise.

Fig 10.14. SalariedEmployee class that implements interface Payable method getPaymentAmount.

 1   // Fig. 10.14: SalariedEmployee.java
 2   // SalariedEmployee class extends Employee, which implements Payable.
 4   public class SalariedEmployee extends Employee
 5   {
 6      private double weeklySalary;
 8      // four-argument constructor
 9      public SalariedEmployee( String first, String last, String ssn,
10         double salary )
11      {
12         super( first, last, ssn ); // pass to Employee constructor
13         setWeeklySalary( salary ); // validate and store salary
14      } // end four-argument SalariedEmployee constructor
16      // set salary
17      public void setWeeklySalary( double salary )
18      {
19         if ( salary >= 0.0 )
20            baseSalary = salary;
21         else
22            throw new IllegalArgumentException(
23               "Weekly salary must be >= 0.0" );
24      } // end method setWeeklySalary
26      // return salary
27      public double getWeeklySalary()
28      {
29         return weeklySalary;
30      } // end method getWeeklySalary
32      // calculate earnings; implement interface Payable method that was
33      // abstract in superclass Employee                                
34      @Override
35      public double getPaymentAmount()                                  
36      {                                                                 
37      return getWeeklySalary();                                      
38      } // end method getPaymentAmount                                  
40      // return String representation of SalariedEmployee object
41      @Override
42      public String toString()
43      {
44         return String.format( "salaried employee: %s\n%s: $%,.2f",
45            super.toString(), "weekly salary", getWeeklySalary() );
46      } // end method toString
47   } // end class SalariedEmployee

When a class implements an interface, the same is-a relationship provided by inheritance applies. Class Employee implements Payable, so we can say that an Employee is a Payable. In fact, objects of any classes that extend Employee are also Payable objects. SalariedEmployee objects, for instance, are Payable objects. Objects of any subclasses of the class that implements the interface can also be thought of as objects of the interface type. Thus, just as we can assign the reference of a SalariedEmployee object to a superclass Employee variable, we can assign the reference of a SalariedEmployee object to an interface Payable variable. Invoice implements Payable, so an Invoice object also is a Payable object, and we can assign the reference of an Invoice object to a Payable variable.

10.7.6 Using Interface Payable to Process Invoices and Employees Polymorphically

PayableInterfaceTest (Fig. 10.15) 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 Payableall 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.

Fig 10.15. Payable interface test program processing Invoices and Employees polymorphically.

 1   // Fig. 10.15: PayableInterfaceTest.java
 2   // Tests interface Payable.
 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 ];
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 );
19         System.out.println(
20            "Invoices and Employees processed polymorphically:\n" );
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 \n%s: $%,.2f\n\n",
27               currentPayable.toString(),
28               "payment due", currentPayable.getPaymentAmount() );
29         } // end for
30      } // end main
31   } // end class PayableInterfaceTest

10.7.7 Common Interfaces of the Java API

In this section, we overview several common interfaces found in the Java API. The power and flexibility of interfaces is used frequently throughout the Java API. These interfaces are implemented and used in the same manner as the interfaces you create (e.g., interface Payable in Section 10.7.2). The Java API's interfaces enable you to use your own classes within the frameworks provided by Java, such as comparing objects of your own types and creating tasks that can execute concurrently with other tasks in the same program. Figure 10.16 overviews a few of the more popular interfaces of the Java API that we use in Java for Programmers, 2/e.

Table 10.16. Common interfaces of the Java API.




Java contains several comparison operators (e.g., <, <=, >, >=, ==, !=) that allow you to compare primitive values. However, these operators cannot be used to compare objects. Interface Comparable is used to allow objects of a class that implements the interface to be compared to one another. Interface Comparable is commonly used for ordering objects in a collection such as an array. We use Comparable in Chapter 18, Generic Collections, and Chapter 19, Generic Classes and Methods.


An interface used to identify classes whose objects can be written to (i.e., serialized) or read from (i.e., deserialized) some type of storage (e.g., file on disk, database field) or transmitted across a network. We use Serializable in Chapter 17, Files, Streams and Object Serialization, and Chapter 24, Networking.


Implemented by any class for which objects of that class should be able to execute in parallel using a technique called multithreading (discussed in Chapter 23, Multithreading). The interface contains one method, run, which describes the behavior of an object when executed.

GUI event-listener interfaces

You work with graphical user interfaces (GUIs) every day. In your web browser, you might type the address of a website to visit, or you might click a button to return to a previous site. The browser responds to your interaction and performs the desired task. Your interaction is known as an event, and the code that the browser uses to respond to an event is known as an event handler. In Chapter 14, GUI Components: Part 1, and Chapter 22, GUI Components: Part 2, you'll learn how to build GUIs and event handlers that respond to user interactions. Event handlers are declared in classes that implement an appropriate event-listener interface. Each event-listener interface specifies one or more methods that must be implemented to respond to user interactions.


Contains a set of constants used in GUI programming to position GUI elements on the screen. We explore GUI programming in Chapters 14 and 22.

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