Kĩ thuật lập trình - Object - Oriented programming: Polymorphism

10Object-Oriented Programming: Polymorphism 1One Ring to rule them all, One Ring to find them, One Ring to bring them all and in the darkness bind them. John Ronald Reuel TolkienGeneral propositions do not decide concrete cases.Oliver Wendell HolmesA philosopher of imposing stature doesn’t think in a vacuum. Even his most abstract ideas are, to some extent, conditioned by what is or is not known in the time when he lives.Alfred North Whitehead Why art thou cast down, O my soul?Psalms 42:5 2OBJECTIVESIn this chapter you will learn:The concept of polymorphism.To use overridden methods to effect polymorphism.To distinguish between abstract and concrete classes.To declare abstract methods to create abstract classes.How polymorphism makes systems extensible and maintainable.To determine an object's type at execution time.To declare and implement interfaces.310.1   Introduction 10.2   Polymorphism Examples 10.3   Demonstrating Polymorphic Behavior 10.4   Abstract Classes and Methods 10.5   Case Study: Payroll System Using Polymorphism 10.5.1  Creating Abstract Superclass Employee 10.5.2  Creating Concrete Subclass SalariedEmployee 10.5.3  Creating Concrete Subclass HourlyEmployee 10.5.4  Creating Concrete Subclass CommissionEmployee 10.5.5  Creating Indirect Concrete Subclass BasePlusCommissionEmployee 10.5.6  Demonstrating Polymorphic Processing, Operator instanceof and Downcasting 10.5.7  Summary of the Allowed Assignments Between Superclass and Subclass Variables 10.6   final Methods and Classes 410.7   Case Study: Creating and Using Interfaces 10.7.1  Developing a Payable Hierarchy 10.7.2  Declaring Interface Payable 10.7.3  Creating Class Invoice 10.7.4  Modifying Class Employee to Implement Interface Payable 10.7.5  Modifying Class SalariedEmployee for Use in the Payable Hierarchy 10.7.6  Using Interface Payable to Process Invoices and Employees Polymorphically 10.7.7  Declaring Constants with Interfaces 10.7.8  Common Interfaces of the Java API 10.8   (Optional) GUI and Graphics Case Study: Drawing with Polymorphism 10.9   (Optional) Software Engineering Case Study: Incorporating Inheritance into the ATM System10.10 Wrap-Up 510.1  Introduction PolymorphismEnables “programming in the general”The same invocation can produce “many forms” of resultsInterfacesImplemented by classes to assign common functionality to possibly unrelated classes 610.2  Polymorphism Examples PolymorphismWhen a program invokes a method through a superclass variable, the correct subclass version of the method is called, based on the type of the reference stored in the superclass variableThe same method name and signature can cause different actions to occur, depending on the type of object on which the method is invokedFacilitates adding new classes to a system with minimal modifications to the system’s code7Software Engineering Observation 10.1Polymorphism enables programmers to deal in generalities and let the execution-time environment handle the specifics. Programmers can command objects to behave in manners appropriate to those objects, without knowing the types of the objects (as long as the objects belong to the same inheritance hierarchy). 8Software Engineering Observation 10.2Polymorphism promotes extensibility: Software that invokes polymorphic behavior is independent of the object types to which messages are sent. New object types that can respond to existing method calls can be incorporated into a system without requiring modification of the base system. Only client code that instantiates new objects must be modified to accommodate new types.910.3  Demonstrating Polymorphic Behavior A superclass reference can be aimed at a subclass objectThis is possible because a subclass object is a superclass object as wellWhen invoking a method from that reference, the type of the actual referenced object, not the type of the reference, determines which method is calledA subclass reference can be aimed at a superclass object only if the object is downcasted10OutlinePolymorphismTest.java(1 of 2) Typical reference assignments11OutlinePolymorphismTest.java(2 of 2) Assign a reference to a basePlusCommissionEmployee object to a CommissionEmployee3 variablePolymorphically call basePlusCommissionEmployee’s toString method1210.4  Abstract Classes and Methods Abstract classesClasses that are too general to create real objectsUsed only as abstract superclasses for concrete subclasses and to declare reference variablesMany inheritance hierarchies have abstract superclasses occupying the top few levelsKeyword abstractUse to declare a class abstractAlso use to declare a method abstractAbstract classes normally contain one or more abstract methodsAll concrete subclasses must override all inherited abstract methods1310.4  Abstract Classes and Methods (Cont.)Iterator classTraverses all the objects in a collection, such as an arrayOften used in polymorphic programming to traverse a collection that contains references to objects from various levels of a hierarchy14Software Engineering Observation 10.3An abstract class declares common attributes and behaviors of the various classes in a class hierarchy. An abstract class typically contains one or more abstract methods that subclasses must override if the subclasses are to be concrete. The instance variables and concrete methods of an abstract class are subject to the normal rules of inheritance. 15Common Programming Error 10.1 Attempting to instantiate an object of an abstract class is a compilation error. 16Common Programming Error 10.2Failure to implement a superclass’s abstract methods in a subclass is a compilation error unless the subclass is also declared abstract.17Fig. 10.2 | Employee hierarchy UML class diagram. 18Software Engineering Observation 10.4A subclass can inherit “interface” or “implementation” from a superclass. Hierarchies designed for implementation inheritance tend to have their functionality high in the hierarchy—each new subclass inherits one or more methods that were implemented in a superclass, and the subclass uses the superclass implementations. (cont)19Software Engineering Observation 10.4Hierarchies designed for interface inheritance tend to have their functionality lower in the hierarchy—a superclass specifies one or more abstract methods that must be declared for each concrete class in the hierarchy, and the individual subclasses override these methods to provide subclass-specific implementations. 2010.5.1 Creating Abstract Superclass Employee abstract superclass Employeeearnings is declared abstractNo implementation can be given for earnings in the Employee abstract classAn array of Employee variables will store references to subclass objectsearnings method calls from these variables will call the appropriate version of the earnings method21Fig. 10.3 | Polymorphic interface for the Employee hierarchy classes. 22OutlineEmployee.java(1 of 3) Declare abstract class EmployeeAttributes common to all employees23OutlineEmployee.java(2 of 3) 24OutlineEmployee.java(3 of 3) abstract method earnings has no implementation25OutlineSalariedEmployee.java(1 of 2) Class SalariedEmployee extends class EmployeeCall superclass constructorValidate and set weekly salary valueCall setWeeklySalary method26OutlineSalariedEmployee.java(2 of 2) Override earnings method so SalariedEmployee can be concreteOverride toString methodCall superclass’s version of toString27OutlineHourlyEmployee.java(1 of 2) Class HourlyEmployee extends class EmployeeCall superclass constructorValidate and set hourly wage value28OutlineHourlyEmployee.java(2 of 2) Validate and set hours worked valueOverride earnings method so HourlyEmployee can be concreteOverride toString methodCall superclass’s toString method29OutlineCommissionEmployee.java(1 of 3) Class CommissionEmployee extends class EmployeeCall superclass constructorValidate and set commission rate value30OutlineCommissionEmployee.java(2 of 3) Validate and set the gross sales value31OutlineCommissionEmployee.java(3 of 3) Override earnings method so CommissionEmployee can be concreteOverride toString methodCall superclass’s toString method32OutlineBasePlusCommissionEmployee.java(1 of 2) Class BasePlusCommissionEmployee extends class CommissionEmployeeCall superclass constructorValidate and set base salary value33OutlineBasePlusCommissionEmployee.java(2 of 2) Override earnings methodCall superclass’s earnings methodOverride toString methodCall superclass’s toString method34OutlinePayrollSystemTest.java(1 of 5) 35OutlinePayrollSystemTest.java(2 of 5) Assigning subclass objects to supercalss variablesImplicitly and polymorphically call toString36OutlinePayrollSystemTest.java(3 of 5) If the currentEmployee variable points to a BasePlusCommissionEmployee objectDowncast currentEmployee to a BasePlusCommissionEmployee referenceGive BasePlusCommissionEmployees a 10% base salary bonusPolymorphically call earnings methodCall getClass and getName methods to display each Employee subclass object’s class name37OutlinePayrollSystemTest.java(4 of 5) 38OutlinePayrollSystemTest.java(5 of 5) Same results as when the employees were processed individuallyBase salary is increased by 10%Each employee’s type is displayed3910.5.6 Demonstrating Polymorphic Processing, Operator instanceof and Downcasting Dynamic bindingAlso known as late bindingCalls to overridden methods are resolved at execution time, based on the type of object referencedinstanceof operatorDetermines whether an object is an instance of a certain type40Common Programming Error 10.3Assigning a superclass variable to a subclass variable (without an explicit cast) is a compilation error. 41If at execution time the reference of a subclass object has been assigned to a variable of one of its direct or indirect superclasses, it is acceptable to cast the reference stored in that superclass variable back to a reference of the subclass type. Before performing such a cast, use the instanceof operator to ensure that the object is indeed an object of an appropriate subclass type. Software Engineering Observation 10.5 42Common Programming Error 10.4When downcasting an object, a ClassCastException occurs, if at execution time the object does not have an is-a relationship with the type specified in the cast operator. An object can be cast only to its own type or to the type of one of its superclasses. 4310.5.6 Demonstrating Polymorphic Processing, Operator instanceof and Downcasting (Cont.)DowncastingConvert a reference to a superclass to a reference to a subclassAllowed only if the object has an is-a relationship with the subclassgetClass methodInherited from ObjectReturns an object of type ClassgetName method of class ClassReturns the class’s name4410.5.7 Summary of the Allowed Assignments Between Superclass and Subclass Variables Superclass and subclass assignment rulesAssigning a superclass reference to a superclass variable is straightforwardAssigning a subclass reference to a subclass variable is straightforwardAssigning a subclass reference to a superclass variable is safe because of the is-a relationshipReferring to subclass-only members through superclass variables is a compilation errorAssigning a superclass reference to a subclass variable is a compilation errorDowncasting can get around this error4510.6  final Methods and Classes final methodsCannot be overridden in a subclassprivate and static methods are implicitly finalfinal methods are resolved at compile time, this is known as static bindingCompilers can optimize by inlining the codefinal classesCannot be extended by a subclassAll methods in a final class are implicitly final46Performance Tip 10.1The compiler can decide to inline a final method call and will do so for small, simple final methods. Inlining does not violate encapsulation or information hiding, but does improve performance because it eliminates the overhead of making a method call. 47Common Programming Error 10.5Attempting to declare a subclass of a final class is a compilation error. 48In the Java API, the vast majority of classes are not declared final. This enables inheritance and polymorphism—the fundamental capabilities of object-oriented programming. However, in some cases, it is important to declare classes final—typically for security reasons. Software Engineering Observation 10.64910.7  Case Study: Creating and Using Interfaces InterfacesKeyword interfaceContains only constants and abstract methodsAll fields are implicitly public, static and finalAll methods are implicitly public abstract methodsClasses can implement interfacesThe class must declare each method in the interface using the same signature or the class must be declared abstractTypically used when disparate classes need to share common methods and constantsNormally declared in their own files with the same names as the interfaces and with the .java file-name extension50Good Programming Practice 10.1According to Chapter 9 of the Java Language Specification, it is proper style to declare an interface’s methods without keywords public and abstract because they are redundant in interface method declarations. Similarly, constants should be declared without keywords public, static and final because they, too, are redundant. 51Common Programming Error 10.6Failing to implement any method of an interface in a concrete class that implements the interface results in a syntax error indicating that the class must be declared abstract. 5210.7.1 Developing a Payable Hierarchy Payable interfaceContains method getPaymentAmountIs implemented by the Invoice and Employee classesUML representation of interfacesInterfaces are distinguished from classes by placing the word “interface” in guillemets (« and ») above the interface nameThe relationship between a class and an interface is known as realizationA class “realizes” the methods of an interface53Good Programming Practice 10.2When declaring a method in an interface, choose a method name that describes the method’s purpose in a general manner, because the method may be implemented by a broad range of unrelated classes. 54Fig. 10.10 | Payable interface hierarchy UML class diagram. 55OutlinePayable.java Declare interface PayableDeclare getPaymentAmount method which is implicitly public and abstract 56OutlineInvoice.java(1 of 3) Class Invoice implements interface Payable57OutlineInvoice.java(2 of 3) 58OutlineInvoice.java(3 of 3) Declare getPaymentAmount to fulfill contract with interface Payable5910.7.3 Creating Class Invoice A class can implement as many interfaces as it needsUse a comma-separated list of interface names after keyword implementsExample: public class ClassName extends SuperclassName implements FirstInterface, SecondInterface, 60OutlineEmployee.java(1 of 3) Class Employee implements interface Payable61OutlineEmployee.java(2 of 3) 62OutlineEmployee.java(3 of 3) getPaymentAmount method is not implemented here6310.7.5 Modifying Class SalariedEmployee for Use in the Payable Hierarchy Objects of any subclasses of the class that implements the interface can also be thought of as objects of the interfaceA reference to a subclass object can be assigned to an interface variable if the superclass implements that interface64Inheritance and interfaces are similar in their implementation of the “is-a” relationship. An object of a class that implements an interface may be thought of as an object of that interface type. An object of any subclasses of a class that implements an interface also can be thought of as an object of the interface type. Software Engineering Observation 10.765OutlineSalariedEmployee.java(1 of 2) Class SalariedEmployee extends class Employee (which implements interface Payable)66OutlineSalariedEmployee.java(2 of 2) Declare getPaymentAmount method instead of earnings method67Software Engineering Observation 10.8The “is-a” relationship that exists between superclasses and subclasses, and between interfaces and the classes that implement them, holds when passing an object to a method. When a method parameter receives a variable of a superclass or interface type, the method processes the object received as an argument polymorphically. 68Software Engineering Observation 10.9Using a superclass reference, we can polymorphically invoke any method specified in the superclass declaration (and in class Object). Using an interface reference, we can polymorphically invoke any method specified in the interface declaration (and in class Object). 69OutlinePayableInterfaceTest.java(1 of 2) Declare array of Payable variablesAssigning references to Invoice objects to Payable variablesAssigning references to SalariedEmployee objects to Payable variables70OutlinePayableInterfaceTest.java(2 of 2) Call toString and getPaymentAmount methods polymorphically71Software Engineering Observation 10.10All methods of class Object can be called by using a reference of an interface type. A reference refers to an object, and all objects inherit the methods of class Object.7210.7.7 Declaring Constants with Interfaces Interfaces can be used to declare constants used in many class declarationsThese constants are implicitly public, static and finalUsing a static import declaration allows clients to use these constants with just their names73Software Engineering Observation 10.11It is considered a better programming practice to create sets of constants as enumerations with keyword enum. See Section 6.10 for an introduction to enum and Section 8.9 for additional enum details.74Fig. 10.16 | Common interfaces of the Java API.(Part 1 of 2)75Fig. 10.16 | Common interfaces of the Java API.  (Part 2 of 2)76Fig. 10.17 | MyShape hierarchy. 77Fig. 10.18 | MyShape hierarchy with MyBoundedShape. 78Fig. 10.19 | Attributes and operations of classes BalanceInquiry, Withdrawal and Deposit. 7910.9 (Optional) Software Engineering Case Study: Incorporating Inheritance into the ATM System UML model for inheritanceThe generalization relationshipThe superclass is a generalization of the subclassesThe subclasses are specializations of the superclassTransaction superclassContains the methods and fields BalanceInquiry, Withdrawal and Deposit have in commonexecute methodaccountNumber field80Fig. 10. 20 | 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. 81Fig. 10.21 | Class diagram of the ATM system (incorporating inheritance). Note that abstract class names (e.g., Transaction) appear in italics. 82Software Engineering Observation 10.12A complete class diagram shows all the associations among classes and all the attributes and operations for each class. When the number of class attributes, methods and associations is substantial (as in Fig. 10.21 and Fig. 10.22), a good practice that promotes readability is to divide this information between two class diagrams—one focusing on associations and the other on attributes and methods. 8310.9 (Optional) Software Engineering Case Study: Incorporating Inheritance into the ATM System (Cont.)Incorporating inheritance into the ATM system designIf class A is a generalization of class B, then class B extends class AIf 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 class84Fig. 10.22 | 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 italic 85OutlineWithdrawal.java Subclass Withdrawal extends superclass Transaction86OutlineWithdrawal.java Subclass Withdrawal extends superclass Transaction87Software Engineering Observation 10.13Several UML modeling tools convert UML-based designs into Java code and can speed the implementation process considerably. For more information on these tools, refer to the Internet and Web Resources listed at the end of Section 2.9. 88OutlineTransaction.java(1 of 2) Declare abstract superclass Transaction89OutlineTransaction.java(2 of 2) Declare abstract method execute90