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Liang,Introduction to Java Programming,revised by Dai-kaiyu 1 Chapter 8 Inheritance and Polymorphism 一切皆有缘起.

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Presentation on theme: "Liang,Introduction to Java Programming,revised by Dai-kaiyu 1 Chapter 8 Inheritance and Polymorphism 一切皆有缘起."— Presentation transcript:

1 Liang,Introduction to Java Programming,revised by Dai-kaiyu 1 Chapter 8 Inheritance and Polymorphism 一切皆有缘起

2 Liang,Introduction to Java Programming,revised by Dai-kaiyu 2 Objectives FTo develop a subclass from a superclass through inheritance (§8.2). FTo invoke the superclass’s constructors and methods using the super keyword (§8.3). FTo override methods in the subclass (§8.4). FTo explore the useful methods (equals(Object), hashCode(), toString(), finalize(), clone(), and getClass()) in the Object class (§8.5, §8.11 Optional). FTo comprehend polymorphism, dynamic binding, and generic programming (§8.6). FTo describe casting and explain why explicit downcasting is necessary (§8.7). FTo understand the effect of hiding data fields and static methods (§8.8 Optional). FTo restrict access to data and methods using the protected visibility modifier (§8.9). FTo declare constants, unmodifiable methods, and nonextendable class using the final modifier (§8.10). F To initialize data using initialization blocks and distinguish between instance initialization and static initialization blocks (§8.12 Optioanl).

3 Liang,Introduction to Java Programming,revised by Dai-kaiyu 3 Introduction Object-oriented programming  Inheritance Software reusability Classes are created from existing ones Absorbing attributes and behaviors Adding new capabilities  Polymorphism Enables developers to write programs in general fashion Helps add new capabilities to system

4 Liang,Introduction to Java Programming,revised by Dai-kaiyu 4 Introduction (cont.) Object-oriented programming  Inheritance Subclass inherits from superclass Subclass usually adds instance variables and methods Single vs. multiple inheritance Java does not support multiple inheritance  Interfaces (discussed later) achieve the same effect “Is a” relationship  Composition “Has a” relationship

5 Liang,Introduction to Java Programming,revised by Dai-kaiyu 5 Object relationships Composition Whole-part –Existance of an object relies on another

6 Liang,Introduction to Java Programming,revised by Dai-kaiyu 6 Superclasses and Subclasses “Is a” Relationship  Object “is an” object of another class Rectangle “is a” quadrilateral Class Rectangle inherits from class Quadrilateral  Form tree-like hierarchical structures

7 Liang,Introduction to Java Programming,revised by Dai-kaiyu 7 Superclasses and Subclasses UML Diagram

8 Liang,Introduction to Java Programming,revised by Dai-kaiyu 8 // Cylinder.java: Class definition for describing Cylinder public class Cylinder extends Circle { private double length = 1; /** Return length */ public double getLength() { return length; } /** Set length */ public void setLength(double length) { this.length = length; } /** Return the volume of this cylinder */ public double findVolume() { return findArea() * length; } supertype subtype

9 Liang,Introduction to Java Programming,revised by Dai-kaiyu 9 Cylinder cylinder = new Cylinder(); System.out.println("The length is " + cylinder.getLength()); System.out.println("The radius is " + cylinder.getRadius()); System.out.println("The volume of the cylinder is " + cylinder.findVolume()); System.out.println("The area of the circle is " + cylinder.findArea()); The length is 1.0 The radius is 1.0 The volume of the cylinder is 3.14159 The area of the circle is 3.14159 The output is Pravate data fields and mothods will not be inherited in a subclass

10 Liang,Introduction to Java Programming,revised by Dai-kaiyu 10 An inheritance hierarchy for university CommunityMembers. CommunityMember is a direct superclass of Employee CommunityMember is an indirect superclass of Faculty

11 Liang,Introduction to Java Programming,revised by Dai-kaiyu 11 A portion of a Shape class hierarchy.

12 Liang,Introduction to Java Programming,revised by Dai-kaiyu 12 Using the Keyword super To call a superclass constructor To call a superclass method The keyword super refers to the superclass of the class in which super appears. This keyword can be used in two ways:

13 Liang,Introduction to Java Programming,revised by Dai-kaiyu 13 CAUTION 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.

14 Liang,Introduction to Java Programming,revised by Dai-kaiyu 14 NOTE 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.

15 Liang,Introduction to Java Programming,revised by Dai-kaiyu 15 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,

16 Liang,Introduction to Java Programming,revised by Dai-kaiyu 16 Constructor Chaining public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System.out.println("Faculty's no-arg constructor is invoked"); } class Employee extends Person { public Employee() { this("Invoke Employee’s overloaded constructor"); System.out.println("Employee's no-arg constructor is invoked"); } public Employee(String s) { System.out.println(s); } class Person { public Person() { System.out.println("Person's no-arg constructor is invoked"); } Constructing an instance of a class invokes all the superclasses’ constructors along the inheritance chain. This is called constructor chaining.

17 Liang,Introduction to Java Programming,revised by Dai-kaiyu 17 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"); } Constructing an instance of a class invokes all the superclasses’ constructors along the inheritance chain. This is called constructor chaining.

18 Liang,Introduction to Java Programming,revised by Dai-kaiyu 18 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); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 1. Start from the main method

19 Liang,Introduction to Java Programming,revised by Dai-kaiyu 19 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); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 2. Invoke Faculty constructor

20 Liang,Introduction to Java Programming,revised by Dai-kaiyu 20 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); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 3. Invoke Employee’s no- arg constructor

21 Liang,Introduction to Java Programming,revised by Dai-kaiyu 21 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); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 4. Invoke Employee(String) constructor

22 Liang,Introduction to Java Programming,revised by Dai-kaiyu 22 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); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 5. Invoke Person() constructor

23 Liang,Introduction to Java Programming,revised by Dai-kaiyu 23 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); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 6. Execute println

24 Liang,Introduction to Java Programming,revised by Dai-kaiyu 24 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); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 7. Execute println

25 Liang,Introduction to Java Programming,revised by Dai-kaiyu 25 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); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 8. Execute println

26 Liang,Introduction to Java Programming,revised by Dai-kaiyu 26 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); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 9. Execute println

27 Liang,Introduction to Java Programming,revised by Dai-kaiyu 27 Example on the Impact of a Superclass without no- arg Constructor public class Apple extends Fruit { } class Fruit { public Fruit(String name) { System.out.println("Fruit's constructor is invoked"); } Find out the errors in the program: 1 、 a default no-arg constructor will be added only when no constructor are specified explicitly. (so Fruit class has no default constructor) 2 、 a class will call the default constructor of its superclass if no explicit calling(so Apple called the default constructor of Fruit,but Fruit class has none such a default one)

28 Liang,Introduction to Java Programming,revised by Dai-kaiyu 28 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

29 Liang,Introduction to Java Programming,revised by Dai-kaiyu 29 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. // Cylinder.java: New cylinder class that overrides the findArea() // method defined in the circle class. public class Cylinder extends Circle { /** Return the surface area of this cylinder. The formula is * 2 * circle area + cylinder body area */ public double findArea() { return 2 * super.findArea() + 2 * getRadius() * Math.PI * length; } // Other methods are omitted }

30 Liang,Introduction to Java Programming,revised by Dai-kaiyu 30 Override vs Overload Overload: same name different signature (by parameters) Can have different return type Can have different modifier Compile error if two methods differ only in return type

31 Liang,Introduction to Java Programming,revised by Dai-kaiyu 31 A Quiz public class Sample{ public void amethod(int i, String s){} } Which ones can be added into the class A ) public void amethod(String s, int i) { } () B ) public int amethod(int i, String s) {return 0;} () C ) private void amethod(int i, String mystring ){ } ( can’t ) D ) public void Amethod(int i, String s) { } ( can ) Can can’t Can

32 Liang,Introduction to Java Programming,revised by Dai-kaiyu 32 Override vs Overload Override  Same method signature and return type  Sub class can’t narrow the access right of the method comparing to super class  Static method can’t be overridden as non-static  Non-static method can’t be overridden as static  Private method can’t be inherited, so can’t be overridden

33 Liang,Introduction to Java Programming,revised by Dai-kaiyu 33 A Quiz which ones is correct public class Base { public void method() {…} } public class Sub extends Base{ public int method() { return 0; } public class Base { public void method() {…} } public class Sub extends Base { public void method(){…} public int method(int a) { return 0; } public class Base { public void method() {…} } public class Sub extends Base { private void method() {…} }

34 Liang,Introduction to Java Programming,revised by Dai-kaiyu 34 Note about override 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.

35 Liang,Introduction to Java Programming,revised by Dai-kaiyu 35 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. (more will be introduced later)

36 Liang,Introduction to Java Programming,revised by Dai-kaiyu 36 Relationship between Superclass Objects and Subclass Objects Subclass object  Can be treated as superclass object Reverse is not true Shape is not always a Circle  Every class implicitly extends java.lang.Object

37 Liang,Introduction to Java Programming,revised by Dai-kaiyu 37 The Object Class 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.

38 Liang,Introduction to Java Programming,revised by Dai-kaiyu 38 The toString() method in Object The toString() 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. Cylinder myCylinder = new Cylinder(5.0, 2.0); System.out.println(myCylinder.toString()); The code displays something like Cylinder@15037e5. This message is not very helpful or informative. Usually you should override the toString method so that it returns a digestible string representation of the object. hashcode

39 Liang,Introduction to Java Programming,revised by Dai-kaiyu 39 Polymorphism, Dynamic Binding and Generic Programming public class Test { public static void main(String[] args) { m(new GraduateStudent()); m(new Student()); m(new Person()); m(new Object()); } public static void m(Object x) { System.out.println(x.toString()); } class GraduateStudent extends Student { } class Student extends Person { public String toString() { return "Student"; } class Person extends Object { public String toString() { 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. Which implementation of toString is used will be determined dynamically by the Java Virtual Machine at runtime. This capability is known as dynamic binding.

40 Liang,Introduction to Java Programming,revised by Dai-kaiyu 40 Dynamic Binding Dynamic binding works as follows: Suppose an object o is an instance of classes C 1, C 2,..., C n-1, and C n, where C 1 is a subclass of C 2, C 2 is a subclass of C 3,..., and C n-1 is a subclass of C n. That is, C n is the most general class, and C 1 is the most specific class. In Java, C n is the Object class. If o invokes a method p, the JVM searches the implementation for the method p in C 1, C 2,..., C n-1 and C n, in this order, until it is found. Once an implementation is found, the search stops and the first-found implementation is invoked.

41 Liang,Introduction to Java Programming,revised by Dai-kaiyu 41 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. Static polymorphism Dynamic polymorphism Demo Sub.java

42 Liang,Introduction to Java Programming,revised by Dai-kaiyu 42 Generic Programming public class Test { public static void main(String[] args) { m(new GraduateStudent()); m(new Student()); m(new Person()); m(new Object()); } public static void m(Object x) { System.out.println(x.toString()); } class GraduateStudent extends Student { } class Student extends Person { public String toString() { return "Student"; } class Person extends Object { public String toString() { 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., toString) is determined dynamically.

43 Liang,Introduction to Java Programming,revised by Dai-kaiyu 43 Casting Objects 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. upcasting

44 Liang,Introduction to Java Programming,revised by Dai-kaiyu 44 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; 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. Enclose the target object type in parentheses and place it before the object to be cast, as follows: Student b = (Student)o; // Explicit casting

45 Liang,Introduction to Java Programming,revised by Dai-kaiyu 45 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. Cylinder myCylinder = (Cylinder)myCircle; Apple x = (Apple)fruit; Orange x = (Orange)fruit; downcasting

46 Liang,Introduction to Java Programming,revised by Dai-kaiyu 46 The instanceof Operator Use the instanceof operator to test whether an object is an instance of a class: Circle myCircle = new Circle(); if (myCircle instanceof Cylinder) { Cylinder myCylinder = (Cylinder)myCircle;... }

47 Liang,Introduction to Java Programming,revised by Dai-kaiyu 47 1 // Point.java 2 // Definition of class Point 3 4 public class Point { 5 protected int x, y; // coordinates of Point 6 7 // No-argument constructor 8 public Point() 9 { 10 // implicit call to superclass constructor occurs here 11 setPoint( 0, 0 ); 12 } 13 14 // constructor 15 public Point( int xCoordinate, int yCoordinate ) 16 { 17 // implicit call to superclass constructor occurs here 18 setPoint( xCoordinate, yCoordinate ); 19 } 20 21 // set x and y coordinates of Point 22 public void setPoint( int xCoordinate, int yCoordinate ) 23 { 24 x = xCoordinate; 25 y = yCoordinate; 26 } 27 28 // get x coordinate 29 public int getX() 30 { 31 return x; 32 } 33 protected members prevent clients from direct access (unless clients are Point subclasses or are in same package)

48 Liang,Introduction to Java Programming,revised by Dai-kaiyu 48 34 // get y coordinate 35 public int getY() 36 { 37 return y; 38 } 39 40 // convert into a String representation 41 public String toString() 42 { 43 return "[" + x + ", " + y + "]"; 44 } 45 46 } // end class Point

49 Liang,Introduction to Java Programming,revised by Dai-kaiyu 49 1 // Fig. 9.5: Circle.java 2 // Definition of class Circle 3 4 public class Circle extends Point { // inherits from Point 5 protected double radius; 6 7 // no-argument constructor 8 public Circle() 9 { 10 // implicit call to superclass constructor occurs here 11 setRadius( 0 ); 12 } 13 14 // constructor 15 public Circle( double circleRadius, int xCoordinate, 16 int yCoordinate ) 17 { 18 // call superclass constructor to set coordinates 19 super( xCoordinate, yCoordinate ); 20 21 // set radius 22 setRadius( circleRadius ); 23 } 24 25 // set radius of Circle 26 public void setRadius( double circleRadius ) 27 { 28 radius = ( circleRadius >= 0.0 ? circleRadius : 0.0 ); 29 } 30 Circle is a Point subclass Implicit call to Point constructor Circle inherits Point ’s protected variables and public methods (except for constuctor) Explicit call to Point constructor using super Error occurs If no non- arg constructor in Point Class Implicit call to Point constructor only apply to the non-arg constructor

50 Liang,Introduction to Java Programming,revised by Dai-kaiyu 50 31 // get radius of Circle 32 public double getRadius() 33 { 34 return radius; 35 } 36 37 // calculate area of Circle 38 public double area() 39 { 40 return Math.PI * radius * radius; 41 } 42 43 // convert the Circle to a String 44 public String toString() 45 { 46 return "Center = " + "[" + x + ", " + y + "]" + 47 "; Radius = " + radius; 48 } 49 50 } // end class Circle Override method toString of class Point by using same signature

51 Liang,Introduction to Java Programming,revised by Dai-kaiyu 51 1 // InheritanceTest.java 2 // Demonstrating the "is a" relationship 3 4 // Java core packages 5 import java.text.DecimalFormat; 6 7 // Java extension packages 8 import javax.swing.JOptionPane; 9 10 public class InheritanceTest { 11 12 // test classes Point and Circle 13 public static void main( String args[] ) 14 { 15 Point point1, point2; 16 Circle circle1, circle2; 17 18 point1 = new Point( 30, 50 ); 19 circle1 = new Circle( 2.7, 120, 89 ); 20 21 String output = "Point point1: " + point1.toString() + 22 "\nCircle circle1: " + circle1.toString(); 23 24 // use "is a" relationship to refer to a Circle 25 // with a Point reference 26 point2 = circle1; // assigns Circle to a Point reference 27 28 output += "\n\nCircle circle1 (via point2 reference): " + 29 point2.toString(); 30 31 // use downcasting (casting a superclass reference to a 32 // subclass data type) to assign point2 to circle2 33 circle2 = ( Circle ) point2; 34 Instantiate Point and Circle objects Circle invokes its overridden toString method Superclass object can reference subclass object Point still invokes Circle ’s overridden toString method Downcast Point to Circle

52 Liang,Introduction to Java Programming,revised by Dai-kaiyu 52 35 output += "\n\nCircle circle1 (via circle2): " + 36 circle2.toString(); 37 38 DecimalFormat precision2 = new DecimalFormat( "0.00" ); 39 output += "\nArea of c (via circle2): " + 40 precision2.format( circle2.area() ); 41 42 // attempt to refer to Point object with Circle reference 43 if ( point1 instanceof Circle ) { 44 circle2 = ( Circle ) point1; 45 output += "\n\ncast successful"; 46 } 47 else 48 output += "\n\npoint1 does not refer to a Circle"; 49 50 JOptionPane.showMessageDialog( null, output, 51 "Demonstrating the \"is a\" relationship", 52 JOptionPane.INFORMATION_MESSAGE ); 53 54 System.exit( 0 ); 55 } 56 57 } // end class InheritanceTest Circle invokes its overridden toString method Circle invokes method areaUse instanceof to determine if Point refers to Circle If Point refers to Circle, cast Point as Circle

53 Liang,Introduction to Java Programming,revised by Dai-kaiyu 53 Assigning subclass references to superclass references

54 Liang,Introduction to Java Programming,revised by Dai-kaiyu 54 Example 8.1 Demonstrating Polymorphism and Casting This example creates two geometric objects: a circle, and a cylinder, invokes the displayGeometricObject method to display the objects. The displayGeometricObject displays the area and perimeter if the object is a circle, and displays area and volume if the object is a cylinder. TestPolymorphismCastingRun

55 Liang,Introduction to Java Programming,revised by Dai-kaiyu 55 Hiding Fields and Static Methods (Optional) You can override an instance method, but you cannot override a field (instance or static) or a static method. If you declare a field or a static method in a subclass with the same name as one in the superclass, the one in the superclass is hidden, but it still exists. The two fields or static methods are independent. You can reference the hidden field or static method using the super keyword in the subclass. The hidden field or method can also be accessed via a reference variable of the superclass’s type.

56 Liang,Introduction to Java Programming,revised by Dai-kaiyu 56 Hiding Fields and Static Methods, cont. When invoking an instance method from a reference variable, the actual class of the object referenced by the variable decides which implementation of the method is used at runtime. When accessing a field or a static method, the declared type of the reference variable decides which method is used at compilation time. Demo SubStatic.java

57 Liang,Introduction to Java Programming,revised by Dai-kaiyu 57 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

58 Liang,Introduction to Java Programming,revised by Dai-kaiyu 58 Accessibility Summary

59 Liang,Introduction to Java Programming,revised by Dai-kaiyu 59 Visibility Modifiers

60 Liang,Introduction to Java Programming,revised by Dai-kaiyu 60 A Subclass Cannot Weaken the Accessibility a subclass cannot weaken the accessibility of a method defined in the superclass. public class Base { public void method() {…} } public class Sub extends Base { private void method() {…} } Base base=new Sub(); base.method(); If above is admitted, then… conflict (compile correct,but runtime error, private can’t be accessed)

61 Liang,Introduction to Java Programming,revised by Dai-kaiyu 61 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. 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.

62 Liang,Introduction to Java Programming,revised by Dai-kaiyu 62 The equals() and hashCode() Methods in the Object Class The equals() method compares the contents of two objects. The hashCode() method returns the hash code of the object. Hash code is an integer, which can be used to store the object in a hash set so that it can be located quickly. Optional

63 Liang,Introduction to Java Programming,revised by Dai-kaiyu 63 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; }

64 Liang,Introduction to Java Programming,revised by Dai-kaiyu 64 NOTE: == vs. equals 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.

65 Liang,Introduction to Java Programming,revised by Dai-kaiyu 65 The hashCode() method The hashCode implemented in the Object class returns the internal memory address of the object in hexadecimal. Your class should override the hashCode method whenever the equals method is overridden. By contract, if two objects are equal, their hash codes must be same. Demo TestHashcode.java

66 Liang,Introduction to Java Programming,revised by Dai-kaiyu 66 The finalize, clone, and getClass Methods The finalize method is invoked by the garbage collector on an object when the object becomes garbage. The clone() method copies an object. The getClass() method returns an instance of the java.lang.Class class, which contains the information about the class for the object. Before an object is created, its defining class is loaded and the JVM automatically creates an instance of java.lang.Class for the class. From this instance, you can discover the information about the class at runtime. Optional java.lang.Class.getName() Reflection Demo frontend TestHashcode.java Circle.java

67 Liang,Introduction to Java Programming,revised by Dai-kaiyu 67 Finalizers Garbage collection  Returns memory to system  Java performs this automatically object marked for garbage collection if no references to object Finalizer method  Returns resources to system  Java provides method finalize Defined in java.lang.Object Receives no parameters Returns void

68 Liang,Introduction to Java Programming,revised by Dai-kaiyu 68 1 // Employee.java 2 // Employee class definition. 3 public class Employee extends Object { 4 private String firstName; 5 private String lastName; 6 private static int count; // number of objects in memory 7 8 // initialize employee, add 1 to static count and 9 // output String indicating that constructor was called 10 public Employee( String first, String last ) 11 { 12 firstName = first; 13 lastName = last; 14 15 ++count; // increment static count of employees 16 System.out.println( "Employee object constructor: " + 17 firstName + " " + lastName ); 18 } 19 20 // subtract 1 from static count when garbage collector 21 // calls finalize to clean up object and output String 22 // indicating that finalize was called 23 protected void finalize() 24 { 25 --count; // decrement static count of employees 26 System.out.println( "Employee object finalizer: " + 27 firstName + " " + lastName + "; count = " + count ); 28 } 29 30 // get first name 31 public String getFirstName() 32 { 33 return firstName; 34 } 35 Employee objects share one instance of count Called when Employee is marked for garbage collection

69 Liang,Introduction to Java Programming,revised by Dai-kaiyu 69 36 // get last name 37 public String getLastName() 38 { 39 return lastName; 40 } 41 42 // static method to get static count value 43 public static int getCount() 44 { 45 return count; 46 } 47 48 } // end class Employee static method accesses static variable count

70 Liang,Introduction to Java Programming,revised by Dai-kaiyu 70 1 // EmployeeTest.java 2 // Test Employee class with static class variable, 3 // static class method, and dynamic memory. 4 import javax.swing.*; 5 6 public class EmployeeTest { 7 8 // test class Employee 9 public static void main( String args[] ) 10 { 11 // prove that count is 0 before creating Employees 12 String output = "Employees before instantiation: " + 13 Employee.getCount(); 14 15 // create two Employees; count should be 2 16 Employee e1 = new Employee( "Susan", "Baker" ); 17 Employee e2 = new Employee( "Bob", "Jones" ); 18 19 // Prove that count is 2 after creating two Employees. 20 // Note: static methods should be called only via the 21 // class name for the class in which they are defined. 22 output += "\n\nEmployees after instantiation: " + 23 "\nvia e1.getCount(): " + e1.getCount() + 24 "\nvia e2.getCount(): " + e2.getCount() + 25 "\nvia Employee.getCount(): " + Employee.getCount(); 26 27 // get names of Employees 28 output += "\n\nEmployee 1: " + e1.getFirstName() + 29 " " + e1.getLastName() + "\nEmployee 2: " + 30 e2.getFirstName() + " " + e2.getLastName(); 31 EmployeeTest can invoke Employee static method, even though Employee has not been instantiated

71 Liang,Introduction to Java Programming,revised by Dai-kaiyu 71 32 // If there is only one reference to each employee (as 33 // on this example), the following statements mark 34 // those objects for garbage collection. Otherwise, 35 // these statement simply decrement the reference count 36 // for each object. 37 e1 = null; 38 e2 = null; 39 40 System.gc(); // suggest call to garbage collector 41 42 // Show Employee count after calling garbage collector. 43 // Count displayed may be 0, 1 or 2 depending on 44 // whether garbage collector executed immediately and 45 // number of Employee objects it collects. 46 output += "\n\nEmployees after System.gc(): " + 47 Employee.getCount(); 48 49 JOptionPane.showMessageDialog( null, output, 50 "Static Members and Garbage Collection", 51 JOptionPane.INFORMATION_MESSAGE ); 52 53 System.exit( 0 ); 54 } 55 56 } // end class EmployeeTest Employee object constructor: Susan Baker Employee object constructor: Bob Jones Employee object finalizer: Susan Baker; count = 1 Employee object finalizer: Bob Jones; count = 0 Calls Java’s automatic garbage- collection mechanism

72 Liang,Introduction to Java Programming,revised by Dai-kaiyu 72

73 Liang,Introduction to Java Programming,revised by Dai-kaiyu 73 Initialization Block Initialization blocks can be used to initialize objects along with the constructors. An initialization block is a block of statements enclosed inside a pair of braces. An initialization block appears within the class declaration, but not inside methods or constructors. It is executed as if it were placed at the beginning of every constructor in the class.

74 Liang,Introduction to Java Programming,revised by Dai-kaiyu 74 Static Initialization Block A static initialization block is much like a nonstatic initialization block except that it is declared static, can only refer to static members of the class, and is invoked when the class is loaded. The JVM loads a class when it is needed. A superclass is loaded before its subclasses.

75 Liang,Introduction to Java Programming,revised by Dai-kaiyu 75 Static Initialization Block class SubStaticInitial extends SuperStaticInitial { static { System.out.println("sub's static initialization block " + "is invoked"); } public static void main(String args[]) { SubStaticInitial test = new SubStaticInitial(); } class SuperStaticInitial { static { System.out.println("super's static initialization block " + "is invoked"); } } Demo SubStaticInitial.java

76 Liang,Introduction to Java Programming,revised by Dai-kaiyu 76 Initialization Block The order of excution  When the class is used for the first time. Load the class,(the superclass is loaded first) initialize static data fields, and excute the static initialization block of the class  When the object is created, constructor is invoked Invoke the constructor of the superclass Initialize instance data fields and execute instance initialization blocks Execute the body of the constructor See the example in the book.


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