Object-Oriented Programming: Polymorphism

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Java for Programmers

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This chapter is from the book

This chapter is from the book 

Java for Programmers

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10.8 (Optional) Software Engineering Case Study: Incorporating Inheritance into the ATM System

We now revisit our ATM system design to see how it might benefit from inheritance. To apply inheritance, we first look for commonality among classes in the system. We create an inheritance hierarchy to model similar (yet not identical) classes in a more elegant and efficient manner. We then modify our class diagram to incorporate the new inheritance relationships. Finally, we demonstrate how our updated design is translated into Java code.

In Section 3.9, we encountered the problem of representing a financial transaction in the system. Rather than create one class to represent all transaction types, we decided to create three individual transaction classes—BalanceInquiry, Withdrawal and Deposit—to represent the transactions that the ATM system can perform. Figure 10.17 shows the attributes and operations of classes BalanceInquiry, Withdrawal and Deposit. Note that these classes have one attribute (accountNumber) and one operation (execute) in common. Each class requires attribute accountNumber to specify the account to which the transaction applies. Each class contains operation execute, which the ATM invokes to perform the transaction. Clearly, BalanceInquiry, Withdrawal and Deposit represent types of transactions. Figure 10.17 reveals commonality among the transaction classes, so using inheritance to factor out the common features seems appropriate for designing classes BalanceInquiry, Withdrawal and Deposit. We place the common functionality in a superclass, Transaction, that classes BalanceInquiry, Withdrawal and Deposit extend.

Fig. 10.17 Attributes and operations of classes BalanceInquiry, Withdrawal and Deposit.

The UML specifies a relationship called a generalization to model inheritance. Figure 10.18 is the class diagram that models the generalization of superclass Transaction and subclasses BalanceInquiry, Withdrawal and Deposit. The arrows with triangular hollow arrowheads indicate that classes BalanceInquiry, Withdrawal and Deposit extend class Transaction. Class Transaction is said to be a generalization of classes BalanceInquiry, Withdrawal and Deposit. Class BalanceInquiry, Withdrawal and Deposit are said to be specializations of class Transaction.

Fig. 10.18 Class diagram modeling generalization of superclass Transaction and subclasses BalanceInquiry, Withdrawal and Deposit. Note that abstract class names (e.g., Transaction) and method names (e.g., execute in class Transaction) appear in italics.

Classes BalanceInquiry, Withdrawal and Deposit share integer attribute accountNumber, so we factor out this common attribute and place it in superclass Transaction. We no longer list accountNumber in the second compartment of each subclass, because the three subclasses inherit this attribute from Transaction. Recall, however, that subclasses cannot access private attributes of a superclass. We therefore include public method getAccountNumber in class Transaction. Each subclass will inherit this method, enabling the subclass to access its accountNumber as needed to execute a transaction.

According to Fig. 10.17, classes BalanceInquiry, Withdrawal and Deposit also share operation execute, so we decided that superclass Transaction should contain public method execute. However, it does not make sense to implement execute in class Transaction, because the functionality that this method provides depends on the type of the actual transaction. We therefore declare method execute as abstract in superclass Transaction. Any class that contains at least one abstract method must also be declared abstract. This forces any subclass of Transaction that must be a concrete class (i.e., BalanceInquiry, Withdrawal and Deposit) to implement method execute. The UML requires that we place abstract class names (and abstract methods) in italics, so Transaction and its method execute appear in italics in Fig. 10.18. Note that method execute is not italicized in subclasses BalanceInquiry, Withdrawal and Deposit. Each subclass overrides superclass Transaction’s execute method with a concrete implementation that performs the steps appropriate for completing that type of transaction. Note that Fig. 10.18 includes operation execute in the third compartment of classes BalanceInquiry, Withdrawal and Deposit, because each class has a different concrete implementation of the overridden method.

Incorporating inheritance provides the ATM with an elegant way to execute all transactions “in the general.” For example, suppose a user chooses to perform a balance inquiry. The ATM sets a Transaction reference to a new object of class BalanceInquiry. When the ATM uses its Transaction reference to invoke method execute, BalanceInquiry’s version of execute is called.

This polymorphic approach also makes the system easily extensible. Should we wish to create a new transaction type (e.g., funds transfer or bill payment), we would just create an additional Transaction subclass that overrides the execute method with a version of the method appropriate for executing the new transaction type. We would need to make only minimal changes to the system code to allow users to choose the new transaction type from the main menu and for the ATM to instantiate and execute objects of the new subclass. The ATM could execute transactions of the new type using the current code, because it executes all transactions polymorphically using a general Transaction reference.

An abstract class like Transaction is one for which the programmer never intends to instantiate objects. An abstract class simply declares common attributes and behaviors of its subclasses in an inheritance hierarchy. Class Transaction defines the concept of what it means to be a transaction that has an account number and executes. You may wonder why we bother to include abstract method execute in class Transaction if it lacks a concrete implementation. Conceptually, we include this method because it corresponds to the defining behavior of all transactions—executing. Technically, we must include method execute in superclass Transaction so that the ATM (or any other class) can polymorphically invoke each subclass’s overridden version of this method through a Transaction reference. Also, from a software engineering perspective, including an abstract method in a superclass forces the implementor of the subclasses to override that method with concrete implementations in the subclasses, or else the subclasses, too, will be abstract, preventing objects of those subclasses from being instantiated.

Subclasses BalanceInquiry, Withdrawal and Deposit inherit attribute accountNumber from superclass Transaction, but classes Withdrawal and Deposit contain the additional attribute amount that distinguishes them from class BalanceInquiry. Classes Withdrawal and Deposit require this additional attribute to store the amount of money that the user wishes to withdraw or deposit. Class BalanceInquiry has no need for such an attribute and requires only an account number to execute. Even though two of the three Transaction subclasses share this attribute, we do not place it in superclass Transaction—we place only features common to all the subclasses in the superclass, otherwise subclasses could inherit attributes (and methods) that they do not need and should not have.

Figure 10.19 presents an updated class diagram of our model that incorporates inheritance and introduces class Transaction. We model an association between class ATM and class Transaction to show that the ATM, at any given moment is either executing a transaction or it is not (i.e., zero or one objects of type Transaction exist in the system at a time). Because a Withdrawal is a type of Transaction, we no longer draw an association line directly between class ATM and class Withdrawal. Subclass Withdrawal inherits superclass Transaction’s association with class ATM. Subclasses BalanceInquiry and Deposit inherit this association, too, so the previously omitted associations between ATM and classes BalanceInquiry and Deposit no longer exist either.

Fig. 10.19 Class diagram of the ATM system (incorporating inheritance). Note that abstract class names (e.g., Transaction) appear in italics.

We also add an association between class Transaction and the BankDatabase (Fig. 10.19). All Transactions require a reference to the BankDatabase so they can access and modify account information. Because each Transaction subclass inherits this reference, we no longer model the association between class Withdrawal and the BankDatabase. Similarly, the previously omitted associations between the BankDatabase and classes BalanceInquiry and Deposit no longer exist.

We show an association between class Transaction and the Screen. All Transactions display output to the user via the Screen. Thus, we no longer include the association previously modeled between Withdrawal and the Screen, although Withdrawal still participates in associations with the CashDispenser and the Keypad. Our class diagram incorporating inheritance also models Deposit and BalanceInquiry. We show associations between Deposit and both the DepositSlot and the Keypad. Note that class BalanceInquiry takes part in no associations other than those inherited from class Transaction—a BalanceInquiry needs to interact only with the BankDatabase and with the Screen.

The class diagram of Fig. 8.21 showed attributes and operations with visibility markers. Now we present a modified class diagram that incorporates inheritance in Fig. 10.20. This abbreviated diagram does not show inheritance relationships, but instead shows the attributes and methods after we have employed inheritance in our system. To save space, as we did in Fig. 4.16, we do not include those attributes shown by associations in Fig. 10.19—we do, however, include them in the Java implementation in Appendix H. We also omit all operation parameters, as we did in Fig. 8.21—incorporating inheritance does not affect the parameters already modeled in Figs. 6.27–6.30.

Fig. 10.20 Class diagram with attributes and operations (incorporating inheritance). Note that abstract class names (e.g., Transaction) and method names (e.g., execute in class Transaction) appear in italics.

Implementing the ATM System Design (Incorporating Inheritance)

In Section 8.18, we began implementing the ATM system design. We now modify our implementation to incorporate inheritance, using class Withdrawal as an example.

1. If a class A is a generalization of class B, then class B extends class A in the class declaration. For example, abstract superclass Transaction is a generalization of class Withdrawal. Figure 10.21 contains the shell of class Withdrawal containing the appropriate class declaration.

   Fig. 10.21. Java code for shell of class Withdrawal.

    1  // Class Withdrawal represents an ATM withdrawal transaction
    2  public class Withdrawal extends Transaction
    3  {
    4  } // end class Withdrawal

2. If class A is an abstract class and class B is a subclass of class A, then class B must implement the abstract methods of class A if class B is to be a concrete class. For example, class Transaction contains abstract method execute, so class Withdrawal must implement this method if we want to instantiate a Withdrawal object. Figure 10.22 is the Java code for class Withdrawal from Fig. 10.19 and Fig. 10.20. Class Withdrawal inherits field accountNumber from superclass Transaction, so Withdrawal does not need to declare this field. Class Withdrawal also inherits references to the Screen and the BankDatabase from its superclass Transaction, so we do not include these references in our code. Figure 10.20 specifies attribute amount and operation execute for class Withdrawal. Line 6 of Fig. 10.22 declares a field for attribute amount. Lines 16–18 declare the shell of a method for operation execute. Recall that subclass Withdrawal must provide a concrete implementation of the abstract method execute in superclass Transaction. The keypad and cashDispenser references (lines 7–8) are fields derived from Withdrawal’s associations in Fig. 10.19. [Note: The constructor in the complete working version of this class will initialize these references to actual objects.]

   Fig. 10.22. Java code for class Withdrawal based on Figs. 10.19 and 10.20.

    1  // Withdrawal.java
    2  // Generated using the class diagrams in Fig. 10.21 and Fig. 10.22
    3  public class Withdrawal extends Transaction
    4  {
    5     // attributes
    6     private double amount; // amount to withdraw
    7     private Keypad keypad; // reference to keypad
    8     private CashDispenser cashDispenser; // reference to cash dispenser
    9
   10     // no-argument constructor
   11     public Withdrawal()
   12     {
   13     } // end no-argument Withdrawal constructor
   14
   15     // method overriding execute
   16     public void execute()
   17     {
   18     } // end method execute
   19  } // end class Withdrawal

Congratulations on completing the design portion of the case study! We completely implement the ATM system in 670 lines of Java code in Appendix H. We recommend that you carefully read the code and its description. The code is abundantly commented and precisely follows the design with which you are now familiar. The accompanying description is carefully written to guide your understanding of the implementation based on the UML design. Mastering this code is a wonderful culminating accomplishment after studying Chapters 1–8.

Software Engineering Case Study Self-Review Exercises

• 10.1 The UML uses an arrow with a __________ to indicate a generalization relationship.

  1. solid filled arrowhead
  2. triangular hollow arrowhead
  3. diamond-shaped hollow arrowhead
  4. stick arrowhead
• 10.2 State whether the following statement is true or false, and if false, explain why: The UML requires that we underline abstract class names and method names.
• 10.3 Write Java code to begin implementing the design for class Transaction specified in Figs. 10.19 and 10.20. Be sure to include private reference-type attributes based on class Transaction’s associations. Also be sure to include public get methods that provide access to any of these private attributes that the subclasses require to perform their tasks.

Answers to Software Engineering Case Study Self-Review Exercises

• 10.1 b.
• 10.2 False. The UML requires that we italicize abstract class names and method names.

• 10.3 The design for class Transaction yields the code in Fig. 10.23. The bodies of the class constructor and methods will be completed in Appendix H. When fully implemented, methods getScreen and getBankDatabase will return superclass Transaction’s private reference attributes screen and bankDatabase, respectively. These methods allow the Transaction subclasses to access the ATM’s screen and interact with the bank’s database.

  Fig. 10.23. Java code for class Transaction based on Figs. 10.19 and 10.20.

   1  // Abstract class Transaction represents an ATM transaction
   2  public abstract class Transaction
   3  {
   4     // attributes
   5     private int accountNumber; // indicates account involved
   6     private Screen screen; // ATM's screen
   7     private BankDatabase bankDatabase; // account info database
   8
   9     // no-argument constructor invoked by subclasses using super()
  10     public Transaction()
  11     {
  12     } // end no-argument Transaction constructor
  13
  14     // return account number
  15     public int getAccountNumber()
  16     {
  17     } // end method getAccountNumber
  18
  19     // return reference to screen
  20     public Screen getScreen()
  21     {
  22     } // end method getScreen
  23
  24     // return reference to bank database
  25     public BankDatabase getBankDatabase()
  26     {
  27     } // end method getBankDatabase
  28
  29     // abstract method overridden by subclasses
  30     public abstract void execute();
  31  } // end class Transaction