Chapter 11 Inheritance and Polymorphism Liang Introduction to

Motivations Suppose you will define classes to model circles, rectangles, and triangles. These classes

Objectives To develop a subclass from a superclass through inheritance (§ 11. 2). To

Superclasses and Subclasses Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education,

// Geometric. Object 1 Class public class Geometric. Object 1 { private String color

public class Circle 4 extends Geometric. Object 1 { private double radius; public Circle

public class Rectangle 1 extends Geometric. Object 1 { private double width; private double

// Test. Circle. Rectangle Main Class public class Test. Circle. Rectangle { public static

Are superclass’s Constructor Inherited? No. They are not inherited. They are invoked explicitly or

Superclass’s Constructor Is Always Invoked A constructor may invoke an overloaded constructor or its

Using the Keyword super The keyword super refers to the superclass of the class

CAUTION You must use the keyword super to call the superclass constructor. Invoking a

Constructor Chaining Constructing an instance of a class invokes all the superclasses’ constructors along

animation Trace Execution public class Faculty extends Employee { public static void main(String[] args)

Example on the Impact of a Superclass without no -arg Constructor Find out the

Declaring a Subclass A subclass extends properties and methods from the superclass. You can

Calling Superclass Methods You could rewrite the print. Circle() method in the Circle class

Overriding Methods in the Superclass A subclass inherits methods from a superclass. Sometimes it

NOTE An instance method can be overridden only if it is accessible. Thus a

NOTE Like an instance method, a static method can be inherited. However, a static

Overriding vs. Overloading Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education,

The Object Class and Its Methods Every class in Java is descended from the

The to. String() method in Object The to. String() method returns a string representation

Polymorphism, Dynamic Binding and Generic Programming public class Polymorphism. Demo { public static void

// Polymorphism. Demo main class public class Polymorphism. Demo { /** Main method */

Dynamic Binding Dynamic binding works as follows: Suppose an object o is an instance

Method Matching vs. Binding Matching a method signature and binding a method implementation are

Generic Programming public class Polymorphism. Demo { public static void main(String[] args) { m(new

Casting Objects You have already used the casting operator to convert variables of one

Why Casting Is Necessary? Suppose you want to assign the object reference o to

Casting from Superclass to Subclass Explicit casting must be used when casting an object

The instanceof Operator Use the instanceof operator to test whether an object is an

TIP To help understand casting, you may also consider the analogy of fruit, apple,

Example: Demonstrating Polymorphism and Casting This example creates two geometric objects: a circle, and

// Example Test. Polymorphism. Casting class public class Test. Polymorphism. Casting { /** Main

The equals Method The equals() method compares the contents of two objects. The default

NOTE The == comparison operator is used for comparing two primitive data type values

The Array. List and Vector Classes You can create an array to store objects.

Compile Warning You will get a compilation warning “unchecked operation. ” Ignore it. This

// Example Test. Array. List class public class Test. Array. List { public static

// Insert a new city at index 2 city. List. add(2, "Xian"); // contains

The My. Stack Classes A stack to hold objects. Liang, Introduction to Java Programming,

// Example Test. Array. List. java public class Test. Array. List { public static

// Example Test. Array. List. java continues … System. out. println("List size? " +

The protected Modifier The protected modifier can be applied on data and methods in

Accessibility Summary Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc.

Visibility Modifiers Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc.

A Subclass Cannot Weaken the Accessibility A subclass may override a protected method in

NOTE The modifiers are used on classes and class members (data and methods), except

The final Modifier The final class cannot be extended: final class Math { .

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Motivations Suppose you will define classes to model circles, rectangles, and triangles. These classes have many common features. What is the best way to design these classes so to avoid redundancy? The answer is to use inheritance. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 2

Objectives To develop a subclass from a superclass through inheritance (§ 11. 2). To invoke the superclass’s constructors and methods using the super keyword (§ 11. 3). To override instance methods in the subclass (§ 11. 4). To distinguish differences between overriding and overloading (§ 11. 5). To explore the to. String() method in the Object class (§ 11. 6). To discover polymorphism and dynamic binding (§§ 11. 7 -11. 8). To describe casting and explain why explicit downcasting is necessary (§ 11. 9). To explore the equals() method in the Object class (§ 11. 10). To store, retrieve, and manipulate objects in an Array. List (§ 11. 11). To implement a Stack class using Array. List (§ 11. 12). To restrict access to data and methods to subclasses using the protected visibility modifier (§ 11. 13). To prevent class extending and method overriding using the final modifier (§ 11. 14). Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 3

// Geometric. Object 1 Class public class Geometric. Object 1 { private String color = "white"; private boolean filled; private java. util. Date date. Created; /** Construct a default geometric object */ public Geometric. Object 1() { date. Created = new java. util. Date(); } /** Construct a geometric object with the specified color and filled value */ public Geometric. Object 1(String Color, boolean filled) { date. Created = new java. util. Date(); this. color = color; this. filled = filled; } /** Return color */ public String get. Color() { return color; } /** Set a new color */ public void set. Color(String color) { this. color = color; } /** Return filled. Since filled is boolean, its get method is named is. Filled */ public boolean is. Filled() { return filled; } /** Set a new filled */ public void set. Filled(boolean filled) { this. filled = filled; } /** Get date. Created */ public java. util. Date get. Date. Created() { return date. Created; } /** Return a string representation of this object */ public String to. String() { return "created on " + date. Created + "ncolor: " + color + " and filled: " + filled; } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 5

public class Circle 4 extends Geometric. Object 1 { private double radius; public Circle 4() { } public Circle 4(double radius) { super(); this. radius = radius; } public Circle 4(double radius, String color, boolean filled) { super(color, filled); this. radius = radius; //set. Color(color); //set. Filled(filled); } /** Return radius */ public double get. Radius() { return radius; } /** Set a new radius */ public void set. Radius(double radius) { this. radius = radius; } /** Return area */ public double get. Area() { return radius * Math. PI; } /** Return diameter */ public double get. Diameter() { return 2 * radius; } /** Return perimeter */ public double get. Perimeter() { return 2 * radius * Math. PI; } /* Print the circle info */ public void print. Circle() { System. out. println(to. String() + "The circle is created " + get. Date. Created() + " and the radius is " + radius); } public String to. String() { return "Circle WWWW " + get. Color() + super. to. String(); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 6

public class Rectangle 1 extends Geometric. Object 1 { private double width; private double height; public Rectangle 1() { } /** Set a new width */ public void set. Width(double width) { this. width = width; } /** Return height */ public double get. Height() { return height; } public Rectangle 1(double width, double height) { this. width = width; this. height = height; } /** Set a new height */ public void set. Height(double height) { this. height = height; } /** Return area */ public double get. Area() { return width * height; } /** Return perimeter */ public double get. Perimeter() { return 2 * (width + height); } public Rectangle 1(double width, double height, String color, boolean filled) { this. width = width; this. height = height; set. Color(color); set. Filled(filled); } /** Return width */ public double get. Width() { return width; } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 7

// Test. Circle. Rectangle Main Class public class Test. Circle. Rectangle { public static void main(String[] args) { Circle 4 circle = new Circle 4(1); System. out. println("A circle " + circle. to. String()); System. out. println("The radius is " + circle. get. Radius()); System. out. println("The area is " + circle. get. Area()); System. out. println("The diameter is " + circle. get. Diameter()); Rectangle 1 rectangle = new Rectangle 1(2, 4); System. out. println("n. A rectangle " + rectangle. to. String()); System. out. println("The area is " + rectangle. get. Area()); System. out. println("The perimeter is " + rectangle. get. Perimeter()); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 8

Are superclass’s Constructor Inherited? No. They are not inherited. They are invoked explicitly or implicitly. Explicitly using the super keyword. A constructor is used to construct an instance of a class. Unlike properties and methods, a superclass's constructors are not inherited in the subclass. They can only be invoked from the subclasses' constructors, using the keyword super. If the keyword super is not explicitly used, the superclass's no-arg constructor is automatically invoked. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 9

Superclass’s Constructor Is Always Invoked A constructor may invoke an overloaded constructor or its superclass’s constructor. If none of them is invoked explicitly, the compiler puts super() as the first statement in the constructor. For example, Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 10

Using the Keyword super The keyword super refers to the superclass of the class in which super appears. This keyword can be used in two ways: To call a superclass constructor To call a superclass method Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 11

CAUTION You must use the keyword super to call the superclass constructor. Invoking a superclass constructor’s name in a subclass causes a syntax error. Java requires that the statement that uses the keyword super appear first in the constructor. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 12

Constructor Chaining Constructing an instance of a class invokes all the superclasses’ constructors along the inheritance chain. This is called constructor chaining. public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System. out. println("(4) Faculty's no-arg constructor is invoked"); } } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System. out. println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System. out. println(s); } } class Person { public Person() { System. out. println("(1) Person's no-arg constructor is invoked"); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 13

animation Trace Execution public class Faculty extends Employee { public static void main(String[] args) { 1. Start from the new Faculty(); } main method public Faculty() { System. out. println("(4) Faculty's no-arg constructor is invoked"); } } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System. out. println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System. out. println(s); } } class Person { public Person() { System. out. println("(1) Person's no-arg constructor is invoked"); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 14

animation Trace Execution public class Faculty extends Employee { public static void main(String[] args) { 2. Invoke Faculty new Faculty(); } constructor public Faculty() { System. out. println("(4) Faculty's no-arg constructor is invoked"); } } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System. out. println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System. out. println(s); } } class Person { public Person() { System. out. println("(1) Person's no-arg constructor is invoked"); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 15

animation Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System. out. println("(4) Faculty's no-arg constructor is invoked"); } } 3. Invoke Employee’s arg constructor class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System. out. println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System. out. println(s); } } class Person { public Person() { System. out. println("(1) Person's no-arg constructor is invoked"); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 no- 16

animation Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System. out. println("(4) Faculty's no-arg constructor is invoked"); } } 4. Invoke Employee(String) class Employee extends Person { constructor public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System. out. println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System. out. println(s); } } class Person { public Person() { System. out. println("(1) Person's no-arg constructor is invoked"); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 17

animation Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System. out. println("(4) Faculty's no-arg constructor is invoked"); } } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System. out. println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System. out. println(s); } 5. Invoke Person() constructor } class Person { public Person() { System. out. println("(1) Person's no-arg constructor is invoked"); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 18

animation Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System. out. println("(4) Faculty's no-arg constructor is invoked"); } } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System. out. println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System. out. println(s); } } 6. Execute println class Person { public Person() { System. out. println("(1) Person's no-arg constructor is invoked"); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 19

animation Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System. out. println("(4) Faculty's no-arg constructor is invoked"); } } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System. out. println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System. out. println(s); } } 7. Execute println class Person { public Person() { System. out. println("(1) Person's no-arg constructor is invoked"); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 20

animation Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System. out. println("(4) Faculty's no-arg constructor is invoked"); } } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System. out. println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System. out. println(s); } } 8. Execute println class Person { public Person() { System. out. println("(1) Person's no-arg constructor is invoked"); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 21

animation Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System. out. println("(4) Faculty's no-arg constructor is invoked"); } } 9. Execute println class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System. out. println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System. out. println(s); } } class Person { public Person() { System. out. println("(1) Person's no-arg constructor is invoked"); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 22

Example on the Impact of a Superclass without no -arg Constructor Find out the errors in the program: public class Apple extends Fruit { } class Fruit { public Fruit(String name) { System. out. println("Fruit's constructor is invoked"); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 23

Declaring a Subclass A subclass extends properties and methods from the superclass. You can also: F Add new properties F Add new methods F Override the methods of the superclass Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 24

Calling Superclass Methods You could rewrite the print. Circle() method in the Circle class as follows: public void print. Circle() { System. out. println("The circle is created " + super. get. Date. Created() + " and the radius is " + radius); } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 25

Overriding Methods in the Superclass A subclass inherits methods from a superclass. Sometimes it is necessary for the subclass to modify the implementation of a method defined in the superclass. This is referred to as method overriding. public class Circle extends Geometric. Object { // Other methods are omitted /** Override the to. String method defined in Geometric. Object */ public String to. String() { return super. to. String() + "nradius is " + radius; } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 26

NOTE An instance method can be overridden only if it is accessible. Thus a private method cannot be overridden, because it is not accessible outside its own class. If a method defined in a subclass is private in its superclass, the two methods are completely unrelated. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 27

NOTE Like an instance method, a static method can be inherited. However, a static method cannot be overridden. If a static method defined in the superclass is redefined in a subclass, the method defined in the superclass is hidden. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 28

The Object Class and Its Methods Every class in Java is descended from the java. lang. Object class. If no inheritance is specified when a class is defined, the superclass of the class is Object. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 30

The to. String() method in Object The to. String() method returns a string representation of the object. The default implementation returns a string consisting of a class name of which the object is an instance, the at sign (@), and a number representing this object. Loan loan = new Loan(); System. out. println(loan. to. String()); The code displays something like Loan@15037 e 5. This message is not very helpful or informative. Usually you should override the to. String method so that it returns a digestible string representation of the object. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 31

Polymorphism, Dynamic Binding and Generic Programming public class Polymorphism. Demo { public static void main(String[] args) { m(new Graduate. Student()); m(new Person()); m(new Object()); } public static void m(Object x) { System. out. println(x. to. String()); } } class Graduate. Student extends Student { } class Student extends Person { public String to. String() { return "Student"; } } class Person extends Object { public String to. String() { return "Person"; } } Method m takes a parameter of the Object type. You can invoke it with any object. An object of a subtype can be used wherever its supertype value is required. This feature is known as polymorphism. When the method m(Object x) is executed, the argument x’s to. String method is invoked. x may be an instance of Graduate. Student, Person, or Object. Classes Graduate. Student, Person, and Object have their own implementation of the to. String method. Which implementation is used will be determined dynamically by the Java Virtual Machine at runtime. This capability is known as dynamic binding. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 32

// Polymorphism. Demo main class public class Polymorphism. Demo { /** Main method */ public static void main(String[] args) { // Display circle and rectangle properties display. Object(new Circle 4(1, "red", false)); display. Object(new Rectangle 1(1, 1, "black", true)); } /** Display geometric object properties */ public static void display. Object(Geometric. Object 1 object) { System. out. println("Created on " + object. get. Date. Created() + ". Color is " + object. get. Color()); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 33

Dynamic Binding Dynamic binding works as follows: Suppose an object o is an instance of classes C 1, C 2, . . . , Cn-1, and Cn, where C 1 is a subclass of C 2, C 2 is a subclass of C 3, . . . , and Cn-1 is a subclass of Cn. That is, Cn is the most general class, and C 1 is the most specific class. In Java, Cn is the Object class. If o invokes a method p, the JVM searches the implementation for the method p in C 1, C 2, . . . , Cn-1 and Cn, in this order, until it is found. Once an implementation is found, the search stops and the first-found implementation is invoked. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 34

Method Matching vs. Binding Matching a method signature and binding a method implementation are two issues. The compiler finds a matching method according to parameter type, number of parameters, and order of the parameters at compilation time. A method may be implemented in several subclasses. The Java Virtual Machine dynamically binds the implementation of the method at runtime. See Review Questions 10. 7 and 10. 9. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 35

Generic Programming public class Polymorphism. Demo { public static void main(String[] args) { m(new Graduate. Student()); m(new Person()); m(new Object()); } public static void m(Object x) { System. out. println(x. to. String()); } } class Graduate. Student extends Student { } class Student extends Person { public String to. String() { return "Student"; } } class Person extends Object { public String to. String() { return "Person"; } } Polymorphism allows methods to be used generically for a wide range of object arguments. This is known as generic programming. If a method’s parameter type is a superclass (e. g. , Object), you may pass an object to this method of any of the parameter’s subclasses (e. g. , Student or String). When an object (e. g. , a Student object or a String object) is used in the method, the particular implementation of the method of the object that is invoked (e. g. , to. String) is determined dynamically. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 36

Casting Objects You have already used the casting operator to convert variables of one primitive type to another. Casting can also be used to convert an object of one class type to another within an inheritance hierarchy. In the preceding section, the statement m(new Student()); assigns the object new Student() to a parameter of the Object type. This statement is equivalent to: Object o = new Student(); // Implicit casting m(o); The statement Object o = new Student(), known as implicit casting, is legal because an instance of Student is automatically an instance of Object. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 37

Why Casting Is Necessary? Suppose you want to assign the object reference o to a variable of the Student type using the following statement: Student b = o; A compilation error would occur. Why does the statement Object o = new Student() work and the statement Student b = o doesn’t? This is because a Student object is always an instance of Object, but an Object is not necessarily an instance of Student. Even though you can see that o is really a Student object, the compiler is not so clever to know it. To tell the compiler that o is a Student object, use an explicit casting. The syntax is similar to the one used for casting among primitive data types. Enclose the target object type in parentheses and place it before the object to be cast, as follows: Student b = (Student)o; // Explicit casting Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 38

Casting from Superclass to Subclass Explicit casting must be used when casting an object from a superclass to a subclass. This type of casting may not always succeed. Apple x = (Apple)fruit; Orange x = (Orange)fruit; Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 39

The instanceof Operator Use the instanceof operator to test whether an object is an instance of a class: Object my. Object = new Circle(); . . . // Some lines of code /** Perform casting if my. Object is an instance of Circle */ if (my. Object instanceof Circle) { System. out. println("The circle diameter is " + ((Circle)my. Object). get. Diameter()); . . . } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 40

TIP To help understand casting, you may also consider the analogy of fruit, apple, and orange with the Fruit class as the superclass for Apple and Orange. An apple is a fruit, so you can always safely assign an instance of Apple to a variable for Fruit. However, a fruit is not necessarily an apple, so you have to use explicit casting to assign an instance of Fruit to a variable of Apple. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 41

Example: Demonstrating Polymorphism and Casting This example creates two geometric objects: a circle, and a rectangle, invokes the display. Geometric. Object method to display the objects. The display. Geometric. Object displays the area and diameter if the object is a circle, and displays area if the object is a rectangle. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 42

// Example Test. Polymorphism. Casting class public class Test. Polymorphism. Casting { /** Main method */ public static void main(String[] args) { // Declare and initialize two objects Object object 1 = new Circle 4(1); Object object 2 = new Rectangle 1(1, 1); // Display circle and rectangle display. Object(object 1); display. Object(object 2); } /** A method for displaying an object */ public static void display. Object(Object object) { if (object instanceof Circle 4) { System. out. println("The circle area is " + ((Circle 4)object). get. Area()); System. out. println("The circle diameter is " + ((Circle 4)object). get. Diameter()); } else if (object instanceof Rectangle 1) { System. out. println("The rectangle area is " + ((Rectangle 1)object). get. Area()); } } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 43

The equals Method The equals() method compares the contents of two objects. The default implementation of the equals method in the Object class is as follows: public boolean equals(Object obj) { return (this == obj); } For example, the equals method is overridden in the Circle class. public boolean equals(Object o) { if (o instanceof Circle) { return radius == ((Circle)o). radius; } else return false; } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 44

NOTE The == comparison operator is used for comparing two primitive data type values or for determining whether two objects have the same references. The equals method is intended to test whether two objects have the same contents, provided that the method is modified in the defining class of the objects. The == operator is stronger than the equals method, in that the == operator checks whether the two reference variables refer to the same object. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 45

The Array. List and Vector Classes You can create an array to store objects. But the array’s size is fixed once the array is created. Java provides the Array. List class that can be used to store an unlimited number of objects. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 46

Compile Warning You will get a compilation warning “unchecked operation. ” Ignore it. This warning can be fixed using generic types in Chapter 20. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 47

// Example Test. Array. List class public class Test. Array. List { public static void main(String[] args) { // Create a list to store cities java. util. Array. List city. List = new java. util. Array. List(); // Add some cities in the list city. List. add("London"); // city. List now contains [London] city. List. add("Denver"); // city. List now contains [London, Denver] city. List. add("Paris"); // city. List now contains [London, Denver, Paris] city. List. add("Miami"); // city. List now contains [London, Denver, Paris, Miami] city. List. add("Seoul"); // contains [London, Denver, Paris, Miami, Seoul] city. List. add("Tokyo"); // contains [London, Denver, Paris, Miami, Seoul, Tokyo] System. out. println("List size? " + city. List. size()); System. out. println("Is Miami in the list? " + city. List. contains("Miami")); System. out. println("The location of Denver in the list? “ + city. List. index. Of("Denver")); System. out. println("Is the list empty? " + city. List. is. Empty()); // Print false Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 48

// Insert a new city at index 2 city. List. add(2, "Xian"); // contains [London, Denver, Xian, Paris, Miami, Seoul, Tokyo] // Remove a city from the list city. List. remove("Miami"); // contains [London, Denver, Xian, Paris, Seoul, Tokyo] // Remove a city at index 1 city. List. remove(1); // contains [London, Xian, Paris, Seoul, Tokyo] // Display the contents in the list System. out. println(city. List. to. String()); // Display the contents in the list in reverse order for (int i = city. List. size() - 1; i >= 0; i--) System. out. print(city. List. get(i) + " "); System. out. println(); // Create a list to store two circles java. util. Array. List list = new java. util. Array. List(); // Add two circles list. add(new Circle 4(2)); list. add(new Circle 4(3)); // Display the area of the first circle in the list System. out. println("The area of the circle? " + ((Circle 4)list. get(0)). get. Area()); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 49

// Example Test. Array. List. java public class Test. Array. List { public static void main(String[] args) { // Create a list to store cities java. util. Array. List city. List = new java. util. Array. List(); // Add some cities in the list city. List. add("London"); // city. List now contains [London] city. List. add("Denver"); // city. List now contains [London, Denver] city. List. add("Paris"); // city. List now contains [London, Denver, Paris] city. List. add("Miami"); // city. List now contains [London, Denver, Paris, Miami] city. List. add("Seoul"); // contains [London, Denver, Paris, Miami, Seoul] city. List. add("Tokyo"); // contains [London, Denver, Paris, Miami, Seoul, Tokyo] Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 51

// Example Test. Array. List. java continues … System. out. println("List size? " + city. List. size()); System. out. println("Is Miami in the list? " + city. List. contains("Miami")); System. out. println("The location of Denver in the list? " + city. List. index. Of("Denver")); System. out. println("Is the list empty? " + city. List. is. Empty()); // Print false public class My. Stack { private java. util. Array. List list = new java. util. Array. List(); public boolean is. Empty() { return list. is. Empty(); } public int get. Size() { return list. size(); } public Object peek() { return list. get(get. Size() - 1); } public Object pop() { Object o = list. get(get. Size() - 1); list. remove(get. Size() - 1); return o; } public void push(Object o) { list. add(o); } public int search(Object o) { return list. last. Index. Of(o); } /** Override the to. String in the Object class */ public String to. String() { return "stack: " + list. to. String(); } } Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 52

The protected Modifier The protected modifier can be applied on data and methods in a class. A protected data or a protected method in a public class can be accessed by any class in the same package or its subclasses, even if the subclasses are in a different package. private, default, protected, public Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 53

A Subclass Cannot Weaken the Accessibility A subclass may override a protected method in its superclass and change its visibility to public. However, a subclass cannot weaken the accessibility of a method defined in the superclass. For example, if a method is defined as public in the superclass, it must be defined as public in the subclass. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 56

NOTE The modifiers are used on classes and class members (data and methods), except that the final modifier can also be used on local variables in a method. A final local variable is a constant inside a method. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 57

The final Modifier The final class cannot be extended: final class Math { . . . } The final variable is a constant: final static double PI = 3. 14159; The final method cannot be overridden by its subclasses. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807 58