Download Chapter 9: Objects and Classes

Chapter 8 Inheritance and Polymorphism
Prerequisites for Part II
Chapter 5 Arrays
Chapter 6 Objects and Classes
Chapter 7 Strings
You can cover GUI after Chapter 8
Chapter 8 Inheritance and Polymorphism
Chapter 11 Getting Started with GUI Programming
Chapter 9 Abstract Classes and Interfaces
Chapter 12 Event-Driven Programming
Chapter 10 Object-Oriented Modeling
一切皆有缘起
Chapter 15 Exceptions and Assertions
You can cover Exceptions and I/O after Chapter 8
Chapter 16 Simple Input and Output
Liang,Introduction to Java Programming,revised by Dai-kaiyu
1
Objectives
 To develop a subclass from a superclass through inheritance (§8.2).
 To invoke the superclass’s constructors and methods using the super keyword
(§8.3).
 To override methods in the subclass (§8.4).
 To explore the useful methods (equals(Object), hashCode(), toString(), finalize(),
clone(), and getClass()) in the Object class (§8.5, §8.11 Optional).
 To comprehend polymorphism, dynamic binding, and generic programming
(§8.6).
 To describe casting and explain why explicit downcasting is necessary (§8.7).
 To understand the effect of hiding data fields and static methods (§8.8 Optional).
 To restrict access to data and methods using the protected visibility modifier
(§8.9).
 To declare constants, unmodifiable methods, and nonextendable class using the
final modifier (§8.10).
 To initialize data using initialization blocks and distinguish between instance
initialization and static initialization blocks (§8.12 Optioanl).
Liang,Introduction to Java Programming,revised by Dai-kaiyu
2
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
Liang,Introduction to Java Programming,revised by Dai-kaiyu
3
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
Liang,Introduction to Java Programming,revised by Dai-kaiyu
4
Object relationships
Composition
•Whole-part
–Existance of an object relies on another
Liang,Introduction to Java Programming,revised by Dai-kaiyu
5
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
Liang,Introduction to Java Programming,revised by Dai-kaiyu
6
Superclasses and Subclasses
Superclass
Circle
Circle Methods
Circle Data
Inheritance
Subclass
UML Diagram
Cylinder
Circle Methods
Cylinder Methods
Circle Data
Cylinder Data
Superclass
Subclass
Circle
Cylinder
-radius
-length
+getRadius
+setRadius
+findArea
Liang,Introduction to Java Programming,revised by Dai-kaiyu
+getLength
+setLength
+findVolume
7
// Cylinder.java: Class definition for describing Cylinder
public class Cylinder extends Circle {
private double length = 1;
Superclass
/** Return length */
public double getLength() {
return length;
}
supertype
Subclass
subtype
Circle
Cylinder
-radius
-length
+getRadius
+setRadius
+findArea
+getLength
+setLength
+findVolume
/** Set length */
public void setLength(double length) {
this.length = length;
}
/** Return the volume of this cylinder */
public double findVolume() {
return findArea() * length;
}
}
Liang,Introduction to Java Programming,revised by Dai-kaiyu
8
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 output is
The
The
The
The
Pravate data fields and mothods will not
be inherited in a subclass
length is 1.0
radius is 1.0
volume of the cylinder is 3.14159
area of the circle is 3.14159
Liang,Introduction to Java Programming,revised by Dai-kaiyu
9
An inheritance hierarchy for university CommunityMembers.
CommunityMember is a direct
superclass of Employee
CommunityMember
CommunityMember is
an indirect superclass of
Faculty
Employee
Faculty
Administrator
Student
Alumni
Staff
Teacher
Liang,Introduction to Java Programming,revised by Dai-kaiyu
10
A portion of a Shape class hierarchy.
Shape
TwoDimensionalShape
Circle
Square
Triangle
ThreeDimensionalShape
Sphere
Cube
Liang,Introduction to Java Programming,revised by Dai-kaiyu
Tetrahedron
11
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,revised by Dai-kaiyu
12
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.
Liang,Introduction to Java Programming,revised by Dai-kaiyu
13
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.
Liang,Introduction to Java Programming,revised by Dai-kaiyu
14
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,
public Cylinder() {
}
public A(double d) {
// some statements
}
is equivalent to
is equivalent to
public Cylinder() {
super();
}
public A(double d) {
super();
// some statements
}
Liang,Introduction to Java Programming,revised by Dai-kaiyu
15
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("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");
Liang,Introduction to Java Programming,revised by Dai-kaiyu
}
}
16
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,revised by Dai-kaiyu
}
17
Trace Execution
public class Faculty extends Employee {
public static void main(String[] args) {
new Faculty();
}
1. Start from the
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,revised by Dai-kaiyu
18
Trace Execution
public class Faculty extends Employee {
public static void main(String[] args) {
new Faculty();
}
2. Invoke 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,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");
}
3. Invoke Employee’s noarg 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,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");
}
}
4. Invoke Employee(String)
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,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);
}
}
5. Invoke Person() constructor
class Person {
public Person() {
System.out.println("(1) Person's no-arg constructor is invoked");
}
}
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);
}
}
6. Execute println
class Person {
public Person() {
System.out.println("(1) Person's no-arg constructor is invoked");
}
}
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);
}
}
7. Execute println
class Person {
public Person() {
System.out.println("(1) Person's no-arg constructor is invoked");
}
}
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);
}
}
8. Execute println
class Person {
public Person() {
System.out.println("(1) Person's no-arg constructor is invoked");
}
}
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");
}
}
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,revised by Dai-kaiyu
26
Example on the Impact of a Superclass without noarg 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");
}
}
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
27
Fruit,but Fruit classLiang,Introduction
has none
such
a default
one)
to Java
Programming,revised
by Dai-kaiyu
Declaring a Subclass
A subclass extends properties and methods from the
superclass. You can also:

Add new properties

Add new methods

Override the methods of the superclass
Liang,Introduction to Java Programming,revised by Dai-kaiyu
28
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
}
Liang,Introduction to Java Programming,revised by Dai-kaiyu
29
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
Liang,Introduction to Java Programming,revised by Dai-kaiyu
30
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 ）
Liang,Introduction to Java Programming,revised by Dai-kaiyu
Can
can’t
can’t
Can
31
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
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32
A Quiz
which ones is correct
public class Base {
public void method() {…} public class Base {
}
public void method() {…}
}
public class Sub extends Base{
public class Sub extends Base {
public int method() {
public void method(){…}
return 0;
public int method(int a) {
}
return 0;
}
}
}
public class Base {
public void method() {…}
}
public class Sub extends Base {
private void method() {…}
}
Liang,Introduction to Java Programming,revised by Dai-kaiyu
33
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.
Liang,Introduction to Java Programming,revised by Dai-kaiyu
34
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)
Liang,Introduction to Java Programming,revised by Dai-kaiyu
35
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
Liang,Introduction to Java Programming,revised by Dai-kaiyu
36
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.
public class Circle {
...
}
Equivalent
public class Circle extends Object {
...
}
Liang,Introduction to Java Programming,revised by Dai-kaiyu
37
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());
hashcode
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.
Liang,Introduction to Java Programming,revised by Dai-kaiyu
38
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.
Liang,Introduction to Java Programming,revised by Dai-kaiyu
39
Dynamic Binding
Dynamic binding works as follows: Suppose an object o is an
instance of classes C1, C2, ..., Cn-1, and Cn, where C1 is a subclass
of C2, C2 is a subclass of C3, ..., and Cn-1 is a subclass of Cn. That
is, Cn is the most general class, and C1 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 C1, C2, ..., 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.
Cn
Cn-1
.....
C2
C1
Since o is an instance of C1, o is also an
Object
instance of C2, C3, …, Cn-1, and Cn
Liang,Introduction to Java Programming,revised by Dai-kaiyu
40
Method Matching vs. Binding
Matching a method signature and binding a method
implementation are two issues.
Static
polymorphism
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.
Dynamic
polymorphism
Demo Sub.java
Liang,Introduction to Java Programming,revised by Dai-kaiyu
41
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.
Liang,Introduction to Java Programming,revised by Dai-kaiyu
42
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);
upcasting
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,revised by Dai-kaiyu
43
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
Liang,Introduction to Java Programming,revised by Dai-kaiyu
44
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
Liang,Introduction to Java Programming,revised by Dai-kaiyu
45
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;
...
}
Liang,Introduction to Java Programming,revised by Dai-kaiyu
46
1 // Point.java
protected members prevent
2 // Definition of class Point
3
clients from direct access (unless
4 public class Point {
clients are Point subclasses or
5
protected int x, y; // coordinates of Point
are in same package)
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
{
Liang,Introduction to Java Programming,revised
31
return x;
by Dai-kaiyu
32
}
33
47
34
35
36
37
38
39
40
41
42
43
44
45
46 }
// get y coordinate
public int getY()
{
return y;
}
// convert into a String representation
public String toString()
{
return "[" + x + ", " + y + "]";
}
// end class Point
Liang,Introduction to Java Programming,revised
by Dai-kaiyu
48
1 // Fig. 9.5: Circle.java
2 // Definition of class Circle
Circle is a Point subclass
3
4 public class Circle extends Point { // inherits from Point
5
protected double radius;
6
Circle inherits Point’s
7
// no-argument constructor
protected variables and public
8
public Circle()
methods (except for constuctor)
9
{
10
// implicit call to superclass constructor occurs here
11
setRadius( 0 );
12
}
Implicit call to Point constructor
13
14
// constructor
15
public Circle( double circleRadius, int xCoordinate,
16
int yCoordinate )
Error occurs If no non17
{
arg constructor in
18
// call superclass constructor to set coordinates
Point Class
19
super( xCoordinate, yCoordinate );
20
Explicit call to Point
21
// set radius
constructor using super
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
Implicit call to Point constructor only apply to
the non-arg constructor
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49
50 }
// get radius of Circle
public double getRadius()
{
return radius;
}
// calculate area of Circle
public double area()
{
return Math.PI * radius * radius;
}
// convert the Circle to a String
public String toString()
{
return "Center = " + "[" + x + ", " + y + "]" +
"; Radius = " + radius;
}
Override method toString of class
Point by using same signature
// end class Circle
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// InheritanceTest.java
// Demonstrating the "is a" relationship
// Java core packages
import java.text.DecimalFormat;
// Java extension packages
import javax.swing.JOptionPane;
public class InheritanceTest {
// test classes Point and Circle
public static void main( String args[] )
{
Point point1, point2;
Circle circle1, circle2;
Instantiate Point and Circle objects
point1 = new Point( 30, 50 );
circle1 = new Circle( 2.7, 120, 89 );
Circle invokes its overridden
toString method
String output = "Point point1: " + point1.toString() +
"\nCircle circle1: " + circle1.toString();
// use "is a" relationship to refer to a Circle
// with a Point reference
point2 = circle1; // assigns Circle to a Point reference
output += "\n\nCircle circle1 (via point2 reference): " +
point2.toString();
Superclass object can
reference subclass object
Point still invokes Circle’s
overridden toString method
// use downcasting (casting a superclass reference to a
// subclass data type) to assign point2 to circle2
circle2 = ( Circle ) point2;
Liang,Introduction to Java Programming,revised
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Downcast Point to Circle
51
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output += "\n\nCircle circle1 (via circle2): " + Circle invokes its overridden
36
circle2.toString();
toString method
37
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DecimalFormat precision2 = new DecimalFormat( "0.00" );
39
output += "\nArea of c (via circle2): " +
Circle invokes method area
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;
Use instanceof to determine
45
output += "\n\ncast successful";
if Point refers to Circle
46
}
47
else
48
output += "\n\npoint1 does not refer to a Circle";
49
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JOptionPane.showMessageDialog( null, output,
If Point refers to Circle,
51
"Demonstrating the \"is a\" relationship",
cast Point as Circle
52
JOptionPane.INFORMATION_MESSAGE );
53
54
System.exit( 0 );
55
}
56
57 } // end class InheritanceTest
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Assigning subclass references to superclass references
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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.
TestPolymorphismCasting
Run
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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.
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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
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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
Visibility increases
private, none (if no modifier is used), protected, public
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Accessibility Summary
Modifier
on members
in a class
Accessed
from the
same class
Accessed
from the
same package
Accessed
from a
subclass
Accessed
from a different
package
public
-
protected
default
private
-
-
-
-
-
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Visibility Modifiers
package p1;
public class C1 {
public int x;
protected int y;
int z;
private int u;
public class C2 {
C1 o = new C1();
can access o.x;
can access o.y;
can access o.z;
cannot access o.u;
protected void m() {
}
}
can invoke o.m();
}
package p2;
public class C3
extends C1 {
can access x;
can access y;
can access z;
cannot access u;
public class C4
extends C1 {
can access x;
can access y;
cannot access z;
cannot access u;
can invoke m();
}
public class C5 {
C1 o = new C1();
can access o.x;
cannot access o.y;
cannot access o.z;
cannot access o.u;
can invoke m();
}
Liang,Introduction to Java Programming,revised by Dai-kaiyu
cannot invoke o.m();
}
59
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() {…}
}
If above is admitted,
then…conflict(compile
correct ,but runtime error,
private can’t be
accessed)
Base base=new Sub();
base.method();
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The final Modifier
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.
 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.
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Optional
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.
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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;
}
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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.
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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
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Optional
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.
Demo frontend
java.lang.Class.getName()
Reflection
TestHashcode.java
Circle.java
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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
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1 // Employee.java
2 // Employee class definition.
Employee objects share one
3 public class Employee extends Object {
4
private String firstName;
instance of count
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 );
Called when Employee
18
}
for garbage collection
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;
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}
by Dai-kaiyu
35
is marked
68
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47
48 }
// get last name
public String getLastName()
{
return lastName;
}
static method accesses
static variable count
// static method to get static count value
public static int getCount()
{
return count;
}
// end class Employee
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// EmployeeTest.java
// Test Employee class with static class variable,
// static class method, and dynamic memory.
import javax.swing.*;
EmployeeTest
public class EmployeeTest {
can invoke Employee static
method, even though Employee has not
been instantiated
// test class Employee
public static void main( String args[] )
{
// prove that count is 0 before creating Employees
String output = "Employees before instantiation: " +
Employee.getCount();
// create two Employees; count should be 2
Employee e1 = new Employee( "Susan", "Baker" );
Employee e2 = new Employee( "Bob", "Jones" );
// Prove that count is 2 after creating two Employees.
// Note: static methods should be called only via the
// class name for the class in which they are defined.
output += "\n\nEmployees after instantiation: " +
"\nvia e1.getCount(): " + e1.getCount() +
"\nvia e2.getCount(): " + e2.getCount() +
"\nvia Employee.getCount(): " + Employee.getCount();
// get names of Employees
output += "\n\nEmployee 1: " + e1.getFirstName() +
" " + e1.getLastName() + "\nEmployee 2: " +
e2.getFirstName() + " " + e2.getLastName();
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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;
Calls Java’s automatic garbage38
e2 = null;
collection mechanism
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
Employee
Employee
Employee
object
object
object
object
constructor: Susan Baker
constructor: Bob Jones
finalizer: Susan Baker; count = 1
finalizer: Bob Jones; count = 0
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Liang,Introduction to Java Programming,revised
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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.
public class Book {
private static int numOfObjects;
private String title
private int id;
public class Book {
private static int numOfObjects;
private String title;
private int id;
Equivalent
public Book(String title) {
this.title = title;
}
public Book(String title) {
numOfObjects++;
this.title = title;
}
public Book(int id) {
this.id = id;
}
public Book(int id) {
numOfObjects++;
this.id = id;
}
{
numOfObjects++;
}
}
}
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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.
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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

}
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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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