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Chapter 9 Inheritance Java Software Solutions Foundations of Program Design Seventh Edition John Lewis William Loftus Copyright © 2012 Pearson Education, Inc. Inheritance • Inheritance is a fundamental object-oriented design technique used to create and organize reusable classes • Chapter 9 focuses on: – – – – – – deriving new classes from existing classes the protected modifier creating class hierarchies abstract classes indirect visibility of inherited members Overloading 8-2 Outline Creating Subclasses Overriding Methods Class Hierarchies Inheritance and Visibility Designing for Inheritance 8-3 Inheritance • Inheritance allows a software developer to derive a new class from an existing one • The existing class is called the parent class, or superclass, or base class • The derived class is called the child class or subclass • As the name implies, the child inherits characteristics of the parent • That is, the child class inherits the methods and data defined by the parent class 8-4 Inheritance • Inheritance relationships are shown in a UML class diagram using a solid arrow with an unfilled triangular arrowhead pointing to the parent class Vehicle Car • Proper inheritance creates an is-a relationship, meaning the child is a more specific version of the parent 8-5 Inheritance • A programmer can tailor a derived class as needed by adding new variables or methods, or by modifying the inherited ones • Software reuse is a fundamental benefit of inheritance • By using existing software components to create new ones, we capitalize on all the effort that went into the design, implementation, and testing of the existing software 8-6 Deriving Subclasses • In Java, we use the reserved word extends to establish an inheritance relationship class Car extends Vehicle { // class contents } • See Words.java (page 382) • See Book.java (page 385) • See Dictionary.java (page 386) 8-7 The protected Modifier • Visibility modifiers affect the way that class members can be used in a child class • Variables and methods declared with private visibility cannot be referenced by name in a child class • They can be referenced in the child class if they are declared with public visibility -- but public variables violate the principle of encapsulation • There is a third visibility modifier that helps in inheritance situations: protected 8-8 The protected Modifier • The protected modifier allows a child class to reference a variable or method directly in the child class • It provides more encapsulation than public visibility, but is not as tightly encapsulated as private visibility • A protected variable is visible to any class in the same package as the parent class • The details of all Java modifiers are discussed in Appendix E • Protected variables and methods can be shown with a # symbol preceding them in UML diagrams 8-9 Class Diagram for Words(p 386) Book # pages : int + pageMessage() : void Words + main (args : String[]) : void Dictionary - definitions : int + definitionMessage() : void 8-10 The super Reference • Constructors are not inherited, even though they have public visibility • Yet we often want to use the parent's constructor to set up the "parent's part" of the object • The super reference can be used to refer to the parent class, and often is used to invoke the parent's constructor • See Words2.java (page 445) • See Book2.java (page 446) • See Dictionary2.java (page 447) 8-11 The super Reference • A child’s constructor is responsible for calling the parent’s constructor • The first line of a child’s constructor should use the super reference to call the parent’s constructor • The super reference can also be used to reference other variables and methods defined in the parent’s class 8-12 Multiple Inheritance • Java supports single inheritance, meaning that a derived class can have only one parent class • Multiple inheritance allows a class to be derived from two or more classes, inheriting the members of all parents • Collisions, such as the same variable name in two parents, have to be resolved • Java does not support multiple inheritance • In most cases, the use of interfaces gives us aspects of multiple inheritance without the overhead 8-13 Outline Creating Subclasses Overriding Methods Class Hierarchies Inheritance and Visibility Designing for Inheritance 8-14 Overriding Methods • A child class can override the definition of an inherited method in favor of its own • The new method must have the same signature as the parent's method, but can have a different body • The type of the object executing the method determines which version of the method is invoked • See Messages.java (page 450) • See Thought.java (page 451) • See Advice.java (page 452) 8-15 Overriding • A method in the parent class can be invoked explicitly using the super reference • If a method is declared with the final modifier, it cannot be overridden • The concept of overriding can be applied to data and is called shadowing variables • Shadowing variables should be avoided because it tends to cause unnecessarily confusing code 8-16 Overloading vs. Overriding • Overloading deals with multiple methods with the same name in the same class, but with different signatures • Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature • Overloading lets you define a similar operation in different ways for different parameters • Overriding lets you define a similar operation in different ways for different object types 8-17 Outline Creating Subclasses Overriding Methods Class Hierarchies Inheritance and Visibility Designing for Inheritance Inheritance and GUIs The Timer Class 8-18 Class Hierarchies • A child class of one parent can be the parent of another child, forming a class hierarchy Business RetailBusiness KMart Macys ServiceBusiness Kinkos 8-19 Class Hierarchies • Two children of the same parent are called siblings • Common features should be put as high in the hierarchy as is reasonable • An inherited member is passed continually down the line • Therefore, a child class inherits from all its ancestor classes • There is no single class hierarchy that is appropriate for all situations 8-20 The Object Class • A class called Object is defined in the java.lang package of the Java standard class library • All classes are derived from the Object class • If a class is not explicitly defined to be the child of an existing class, it is assumed to be the child of the Object class • Therefore, the Object class is the ultimate root of all class hierarchies 8-21 The Object Class • The Object class contains a few useful methods, which are inherited by all classes • For example, the toString method is defined in the Object class • Every time we define the toString method, we are actually overriding an inherited definition • The toString method in the Object class is defined to return a string that contains the name of the object’s class along with some other information 8-22 The Object Class • The equals method of the Object class returns true if two references are aliases • We can override equals in any class to define equality in some more appropriate way • As we've seen, the String class defines the equals method to return true if two String objects contain the same characters • The designers of the String class have overridden the equals method inherited from Object in favor of a more useful version 8-23 Abstract Classes(page 401) • An abstract class is a placeholder in a class hierarchy that represents a generic concept • An abstract class cannot be instantiated • We use the modifier abstract on the class header to declare a class as abstract: public abstract class Product { // contents } 8-24 Abstract Classes • An abstract class often contains abstract methods with no definitions (like an interface) • Unlike an interface, the abstract modifier must be applied to each abstract method • Also, an abstract class typically contains nonabstract methods with full definitions • A class declared as abstract does not have to contain abstract methods -- simply declaring it as abstract makes it so 8-25 Abstract Classes • The child of an abstract class must override the abstract methods of the parent, or it too will be considered abstract • An abstract method cannot be defined as final or static • The use of abstract classes is an important element of software design – it allows us to establish common elements in a hierarchy that are too generic to instantiate 8-26 Interface Hierarchies • Inheritance can be applied to interfaces as well as classes • That is, one interface can be derived from another interface • The child interface inherits all abstract methods of the parent • A class implementing the child interface must define all methods from both the ancestor and child interfaces • Note that class hierarchies and interface hierarchies are distinct (they do not overlap) 8-27 Outline Creating Subclasses Overriding Methods Class Hierarchies Inheritance and Visibility Designing for Inheritance 8-28 Visibility Revisited • It's important to understand one subtle issue related to inheritance and visibility • All variables and methods of a parent class, even private members, are inherited by its children • As we've mentioned, private members cannot be referenced by name in the child class • However, private members inherited by child classes exist and can be referenced indirectly 8-29 Visibility Revisited • Because the parent can refer to the private member, the child can reference it indirectly using its parent's methods • The super reference can be used to refer to the parent class, even if no object of the parent exists • See FoodAnalyzer.java (page 459) • See FoodItem.java (page 460) • See Pizza.java (page 461) 8-30 Summary • Chapter 09 focused on: – – – – – – deriving new classes from existing classes the protected modifier creating class hierarchies abstract classes indirect visibility of inherited members designing for inheritance 8-31