© 2013 Ken Howard, Southern Methodist University Object-Oriented Programming: Inheritance CSE 1341

© 2013 Ken Howard, Southern Methodist University Abstraction

© 2013 Ken Howard, Southern Methodist University Abstraction

© 2013 Ken Howard, Southern Methodist University Building Church Residence Office School Cathedral

© 2013 Ken Howard, Southern Methodist University Die A die component: Has a face value. Can roll itself — “Do It Myself.” RegularDie -faceValue: int +RegularDie() +getFaceValue(): int +setFaceValue(int):void +roll():int +RegularDie() +getFaceValue(): int +setFaceValue(int):void +roll():int

© 2013 Ken Howard, Southern Methodist University Die A die component: Has a face value. Can roll itself — “Do It Myself.” RegularDie -faceValue: int +Die() +getFaceValue(): int +setFaceValue(int):void +roll():int +Die() +getFaceValue(): int +setFaceValue(int):void +roll():int

© 2013 Ken Howard, Southern Methodist University Extending the Die Class Objects (software components) can have variations on a theme. Consider a new kind of Die, LoadedDie, that can remember a loaded side and set the value of half of the rolls to the loadedSide. LoadedDie -faceValue: int -loadedSide: int -faceValue: int -loadedSide: int +LoadedDie() +getFaceValue(): int +setFaceValue(int):void +getLoadedSide() : int +setLoadedSide(int):void +roll():int +LoadedDie() +getFaceValue(): int +setFaceValue(int):void +getLoadedSide() : int +setLoadedSide(int):void +roll():int

© 2013 Ken Howard, Southern Methodist University Die and LoadedDie: Common Parts LoadedDie -faceValue: int -loadedSide: int +LoadedDie() +getFaceValue(): int +setFaceValue(int):void +getLoadedSide() : int +setLoadedSide(int):void +roll():int RegularDie -faceValue: int +RegularDie() +getFaceValue(): int +setFaceValue(int):void +roll():int +RegularDie() +getFaceValue(): int +setFaceValue(int):void +roll():int

© 2013 Ken Howard, Southern Methodist University Inheritance LoadedDie -loadedSide: int +LoadedDie() +getLoadedSide() : int +setLoadedSide(int):void +roll() : int RegularDie -faceValue: int +RegularDie() +getFaceValue(): int +setFaceValue(int):void +roll():int +RegularDie() +getFaceValue(): int +setFaceValue(int):void +roll():int over-ridden inherited added inherited Superclass Subclass

© 2013 Ken Howard, Southern Methodist University Inheritance LoadedDie -loadedSide: int +LoadedDie() +getLoadedSide() : int +setLoadedSide(int):void +roll() : int RegularDie -faceValue: int +Die() +getFaceValue(): int +setFaceValue(int):void +roll():int +Die() +getFaceValue(): int +setFaceValue(int):void +roll():int Inheritance: – Is present when software classes are organized in a generalization- specialization hierarchy. – Means that a subclass acquires all of the attribute and method definitions of its superclass. A subclass can add its own unique attributes and behaviors. A subclass might also override (redefine) an inherited method. Advantages? over-ridden inherited added inherited

© 2013 Ken Howard, Southern Methodist University Polymorphism Polymorphism means: – The same message can be interpreted in different ways, depending on the receiver. – A method by the same name can be defined in different classes. How do you know which method will be executed when you send a message? Advantages? roll() RegularDie LoadedDie

© 2013 Ken Howard, Southern Methodist University LoadedDie -loadedSide: int +LoadedDie() +getLoadedSide() : int +setLoadedSide(int):void +roll() : int TenSidedDie +TenSidedDie() +roll() : int +TenSidedDie() +roll() : int Die -faceValue: int +Die() +getFaceValue(): int +setFaceValue(int):void +roll():int +Die() +getFaceValue(): int +setFaceValue(int):void +roll():int Abstract Superclass Not instantiated. Superclass exists only to define common parts. Abstract Superclass Not instantiated. Superclass exists only to define common parts. RegularDie +RegularDie()

© 2013 Ken Howard, Southern Methodist University Die LoadedDie RegularDie TenSidedDie

© 2013 Ken Howard, Southern Methodist University Abstract and Concrete Classes Abstract class: Not instantiated. Superclass to define common parts. Concrete class: Instantiated.

© 2013 Ken Howard, Southern Methodist University Domain Modeling: Inheritance Generalizations should be discovered, not predicted. – In analysis, observe common relationships and common attributes. – In design, observe shared behavior for which delegation cannot be used to provide the common implementation. 100% Rule: All statements about the generalization are valid for the specialization. IF you keep Superclasses abstract, they’ll be more stable.

© 2013 Ken Howard, Southern Methodist University Die LoadedDie roll RegularDie

© 2013 Ken Howard, Southern Methodist University Is a… Animal Cat Dog “Bark” “” “Meow ” talk

© 2013 Ken Howard, Southern Methodist University ? talk

© 2013 Ken Howard, Southern Methodist University public abstract class Animal { public abstract String talk(); } public class Dog extends Animal { public String talk() { return “Yip!”; }

© 2013 Ken Howard, Southern Methodist University If I throw objects into a bag…

© 2013 Ken Howard, Southern Methodist University Domain Modeling: Inheritance

© 2013 Ken Howard, Southern Methodist University Polymorphism One request handled by receiver in different ways. - the requestor doesn’t have to get involved in details - the receiver does what it best knows how to do

© 2013 Ken Howard, Southern Methodist University Traditional handling approach with conditional logic: public Money getTotalPrice() { Money price; // calculate prices and tax … if (shipping.equals("Standard")) { price += //calculate standard shipping } else if (shipping.equals("Ground")) { price += //calculate ground shipping } else if (shipping.equals("Priority")) { price += //calculate ground shipping } // FIXME: support Int’l Shipping! return price; }

© 2013 Ken Howard, Southern Methodist University public Date getEstimatedDeliveryDate() { Date dueDate; if (shipping.equals("Standard")) { dueDate = //calculate standard shipping } else if (shipping.equals("Ground")) { dueDate = //calculate ground shipping } else if (shipping.equals("Priority")) { dueDate = //calculate ground shipping } // FIXME: support Courier return dueDate; }

© 2013 Ken Howard, Southern Methodist University What now?

© 2013 Ken Howard, Southern Methodist University Better? public Money getTotalPrice() { Money price; // calculate prices and tax … price += shippingStrategy.getShippingCost(weight, dimensions, destination); return price; } public Date getEstimatedDeliveryDate() { Date dueDate = shippingStrategy.getEstimatedDeliveryDate(); return dueDate; }

© 2013 Ken Howard, Southern Methodist University Polymorphism Example

© 2013 Ken Howard, Southern Methodist University Basic Inheritance Terminology Superclass Subclass Specialization Inheritance Class Hierarchy “Is a” (vs. “Has a”)

© 2013 Ken Howard, Southern Methodist University More Terminology The direct superclass is the superclass from which the subclass explicitly inherits. An indirect superclass is any class above the direct superclass in the class hierarchy. The Java class hierarchy begins with class Object (in package java.lang )  Every class in Java directly or indirectly extends (or “inherits from”) Object. Java supports only single inheritance, in which each class is derived from exactly one direct superclass.

© 2013 Ken Howard, Southern Methodist University Superclasses and Subclasses The figure below lists several simple examples of superclasses and subclasses  Superclasses tend to be “more general” and subclasses “more specific.” Because every subclass object is an object of its superclass, and one superclass can have many subclasses, the set of objects represented by a superclass is typically larger than the set of objects represented by any of its subclasses.

© 2013 Ken Howard, Southern Methodist University

Not every class relationship is an inheritance relationship. Has-a relationship (NOT INHERITANCE)  Create classes by composition of existing classes.  Example: Given the classes Employee, BirthDate and TelephoneNumber, it’s improper to say that an Employee is a BirthDate or that an Employee is a TelephoneNumber.  However, an Employee has a BirthDate, and an Employee has a TelephoneNumber.

© 2013 Ken Howard, Southern Methodist University Objects of all classes that extend a common superclass can be treated as objects of that superclass.  Commonality expressed in the members of the superclass. Inheritance issue  A subclass can inherit methods that it does not need or should not have.  Even when a superclass method is appropriate for a subclass, that subclass often needs a customized version of the method.  The subclass can override (redefine) the superclass method with an appropriate implementation.

© 2013 Ken Howard, Southern Methodist University Slot -amount : int +getScore():int SpinGame +SpinGame() +playGame():void +displayScore():void +SpinGame() +playGame():void +displayScore():void 10 1 GameTester +main(String[]):void slots 11 game Wheel +Wheel() +spin():int +Wheel() +spin():int theWheel 1 1 LoseSlot +getScore():int WinSlot +getScore():int BonusSlot +getScore():int

© 2013 Ken Howard, Southern Methodist University

protected Members A class’s public members are accessible wherever the program has a reference to an object of that class or one of its subclasses. A class’s private members are accessible only within the class itself. protected access is an intermediate level of access between public and private.  A superclass’s protected members can be accessed by members of that superclass, by members of its subclasses and by members of other classes in the same package protected members also have package access.  All public and protected superclass members retain their original access modifier when they become members of the subclass.

© 2013 Ken Howard, Southern Methodist University protected Members (Cont.) A superclass’s private members are hidden in its subclasses  They can be accessed only through the public or protected methods inherited from the superclass Subclass methods can refer to public and protected members inherited from the superclass simply by using the member names. When a subclass method overrides an inherited superclass method, the superclass method can be accessed from the subclass by preceding the superclass method name with keyword super and a dot (. ) separator.

© 2013 Ken Howard, Southern Methodist University Use of private with Inheritance Members of a subclass cannot access private members of its superclass. Preferred: Access the superclass attributes through methods in the superclass. Alternative: Declare superclass attributes as protected.

© 2013 Ken Howard, Southern Methodist University Student -studentNumber: int +Student() +Student(String,int) +getStudentNumber() : int +setStudentNumber(int):void +equalsl(Student) : boolean +toString() : String Person -name : String +Person() +Person(String) +getName(): String +setName(String):void +sameName(Person):boolean +toString():String +Person() +Person(String) +getName(): String +setName(String):void +sameName(Person):boolean +toString():String Another Example:

© 2013 Ken Howard, Southern Methodist University public class Person { private String name; public Person() { name = "No name yet."; } public Person(String initialName) { name = initialName; } public void setName(String newName) { name = newName; } Example: Base Class Base Class public String getName() { return name; } public toString () { return ”Person: " + name; } public boolean sameName(Person otherPerson) { return (this.name.equalsIgnoreCase(otherPerson. name)); }

© 2013 Ken Howard, Southern Methodist University Programming Example: Derived Class Derived Class public class Student extends Person { private int studentNumber; public Student() { super();//super is explained in a later section. studentNumber = 0;//Indicating no number yet } public Student(String initialName, int initialStudentNumber) { super(initialName); studentNumber = initialStudentNumber; } public int getStudentNumber() { return studentNumber; } public void setStudentNumber(int newStudentNumber) { studentNumber = newStudentNumber; } public String toStringt() { return super.toString() + ” Student Number : " + studentNumber; } public boolean equals(Student otherStudent) { return (this.sameName(otherStudent) && (this.studentNumber == otherStudent.studentNumber)); }

© 2013 Ken Howard, Southern Methodist University Programming Example: Test Class Test Class public class InheritanceDemo { public static void main(String[] args) { Student s = new Student(); s.setName("Warren Peace"); //setName is inherited from the class Person. s.setStudentNumber(2001); s.writeOutput(); } Screen Output Name: Warren Peace Student Number: 2001

© 2013 Ken Howard, Southern Methodist University Review… Private attributes cannot be accessed in subclass Private methods are not inherited Overloading: same method name, different parameters Overriding: same method signature implemented in a subclass

© 2013 Ken Howard, Southern Methodist University Also... Constructors are not inherited. The first task of a subclass constructor is to call its direct superclass’s constructor explicitly or implicitly  Ensures that the instance variables inherited from the superclass are initialized properly. If the code does not include an explicit call to the superclass constructor, Java implicitly calls the superclass’s default or no-argument constructor. A class’s default constructor calls the superclass’s default or no-argument constructor.

© 2013 Ken Howard, Southern Methodist University Inheriting from “Object” toString is one of the methods that every class inherits directly or indirectly from class Object.  Returns a String representing an object.  Called implicitly whenever an object must be converted to a String representation. Class Object ’s toString method returns a String that includes the name of the object’s class.  This is primarily a placeholder that can be overridden by a subclass to specify an appropriate String representation.

© 2013 Ken Howard, Southern Methodist University

protected members Using protected instance variables creates several potential problems. The subclass object can set an inherited variable’s value directly without using a set method.  A subclass object can assign an invalid value to the variable, possibly leaving the object in an inconsistent state. Subclass methods are more likely to be written so that they depend on the superclass’s data implementation.  Subclasses should depend only on the superclass services and not on the superclass data implementation.

© 2013 Ken Howard, Southern Methodist University

(C) 2010 Pearson Education, Inc. All rights reserved.

© 2013 Ken Howard, Southern Methodist University

public void foo() { foo(); doSomething(); doSomethingElse(); } public void foo() { super.foo(); doSomething(); doSomethingElse(); } VS.

© 2013 Ken Howard, Southern Methodist University 9.6 Constructors in Subclasses Instantiating a subclass object begins a chain of constructor calls  The subclass constructor, before performing its own tasks, invokes its direct superclass’s constructor If the superclass is derived from another class, the superclass constructor invokes the constructor of the next class up the hierarchy, and so on. The last constructor called in the chain is always class Object ’s constructor. Original subclass constructor’s body finishes executing last. Each superclass’s constructor manipulates the superclass instance variables that the subclass object inherits.

© 2013 Ken Howard, Southern Methodist University

Software Engineering with Inheritance When you extend a class, the new class inherits the superclass’s members—though the private superclass members are hidden in the new class. You can customize the new class to meet your needs by including additional members and by overriding superclass members.  Doing this does not require the subclass programmer to change (or even have access to) the superclass’s source code.  Java simply requires access to the superclass’s.class file.

© 2013 Ken Howard, Southern Methodist University Object Class All classes in Java inherit directly or indirectly from Object, so its 11 methods are inherited by all other classes. Figure 9.12 summarizes Object ’s methods. Can learn more about Object ’s methods in the online API documentation and in The Java Tutorial at : java.sun.com/javase- /6/docs/api/java/lang/Object.html or java.sun.com/docs/books/tutorial/java/IandI/ objectclass.html Every array has an overridden clone method that copies the array.  If the array stores references to objects, the objects are not copied—a shallow copy is performed. For more information about the relationship between arrays and class Object, see Java Language Specification, Chapter 10, at java.sun.com/docs/books/jls/third_edition/ html/arrays.html