Download Chapter 9: Objects and Classes

Chapter 3 Inheritance and Polymorphism
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To develop a subclass from a superclass through inheritance.
To invoke the superclass’s constructors and methods using the super
keyword.
To override methods in the subclass.
To distinguish differences between overriding and overloading.
To explore the toString() method in the Object class.
To discover polymorphism, dynamic binding.
To explore the equals() method in the Object class.
To store, retrieve, and manipulates objects in an ArrayList.
To implement a Stack class using ArrayList.
To restrict access to data and methods using the protected visibility
modifier.
To prevent class extending and method overriding using the final
modifier.
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1
Inheritance
The reusability can be obtained by creating new classes and
reusing the properties of the existing one.

The mechanism of deriving a new class from existing one is
called as inheritance. The old class or existing class is known
as super class and new class is known as sub class.
Creating the inheritance
The class can derived from another class by following the syntax:
class subclassname extends superclassname
{
//Body of the class;
}
Note: The keyword ‘extends’ specifies that the properties of
superclassname are extended to subclassname. After this the
subclass will contain all the methods of super class and it will
add the members of its own.

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//Simple Example
5)class Employee{
6) float salary=40000;
}
4)8)class Programmer extends Employee{
9)int bonus=10000;
1) public static void main(String args[])
{
2)Programmer p=new Programmer();
3)System.out.println("Programmer salary is:"+p.salary);
7)System.out.println("Bonus of Programmer is:"+p.bonus);
}
}
Output:
Programmer salary is:40000.0
Bonus of programmer is:10000
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// An example of Single inheritance 14)return mem;
import java.util.Scanner;
}
class First{
}
int val;
class SingleInheritance
5)void init(){
{
6)System.out.print("Enter a number: "); 1)public static void main(String args[])
Scanner in = new Scanner(System.in); {
7)val = in.nextInt();
2)Second s = new Second();
}
3)s.init();
11)int square(){
8)System.out.println("Cube :
"+s.cube());
12)return(val*val);}
}
}
}
4)class Second extends First{
Output:
int mem;
Example of single inheritance
9)int cube(){
Enter a number: 5
10)13)mem = square() * val;
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Cube : 125
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Superclasses and Subclasses
GeometricObject
-color: String
The color of the object (default: white).
-filled: boolean
Indicates whether the object is filled with a color (default: false).
-dateCreated: java.util.Date
The date when the object was created.
+GeometricObject()
Creates a GeometricObject.
+getColor(): String
Returns the color.
+setColor(color: String): void
Sets a new color.
+isFilled(): boolean
Returns the filled property.
+setFilled(filled: boolean): void
Sets a new filled property.
+getDateCreated(): java.util.Date
Returns the dateCreated.
+toString(): String
Returns a string representation of this object.
GeometricObject
Circle
Rectangle
Circle
-radius: double
-width: double
+Circle()
-height: double
+Circle(radius: double)
+Rectangle()
+getRadius(): double
+Rectangle(width: double, height: double)
+setRadius(radius: double): void
+getWidth(): double
+getArea(): double
+setWidth(width: double): void
+getPerimeter(): double
+getHeight(): double
+getDiameter(): double
+setHeight(height: double): void
Rectangle
TestCircleRectangle
+getArea(): double
Run
+getPerimeter(): double
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// A simple example of inheritance.
a.i = 5;
class Super{
a.j = 12;
int i, j;
System.out.println("Contents of super: ");
void show(){
a.show(); System.out.println();
System.out.print("i and j: ");
b.i = 11; b.j = 13; b.k = 17;
System.out.println( + i + " " + j);}
System.out.println("Contents of sub: ");
}
b.show(); b.display(); System.out.println();
class Sub extends Super{
System.out.println("Sum of i, j and k in
sub:");
int k;
b.sum();
void display(){
}} Output:
System.out.println("k: " + k);}
Contents of super:
void sum(){
i and j: 5 12
System.out.println("i+j+k: " + (i+j+k));}
Contents of sub:
}
i and j: 11 13
class SimpleInheritance{
k: 17
public static void main(String args[]){
Sum of i, j and k in sub:
Super a = new Super();
i+j+k: 41
Sub b = new Sub();
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10
Superclasses and Subclasses
/* Super class variable to
class Reference{
Output:
subclass object*/
public static void main(String
Super class variable can
class Super{
args[]){
refer to the subclass object
int i, j;
Super a; //Statement1
Contents of sub:
void show(){
Sub b = new Sub();
i and j: 11 13
System.out.print("i and j: ");
b.i = 11; b.j = 13; b.k = 17;
k: 17
System.out.println( + i + " "
System.out.println("Contents of
Contents of super:
+ j);}
sub: ");
i and j: 11 13
}
b.show(); b.display();
Sum of i, j and k in sub:
System.out.println();
class Sub extends Super{
i+j+k: 41
a = b; //Statement2
int k;
System.out.println("Contents of
void display(){
super: ");
System.out.println("k: " + k);
a.show(); System.out.println();
}
System.out.println("Sum of i, j
void sum(){
and k in sub:");
System.out.println("i+j+k: “
b.sum();
+ (i+j+k));
}
}
}
}
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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:
Accessing members from super class that has been
hidden by members of the subclass
super.variablename;
 Calling super class constructor
super(argumentlist);

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Accessing Members from super class
11)System.out.println("Sub class cal :
"+cal);
//Use of super to access super class
members
}
class Primary
}
{
Class SuperUse1
int cal; //declaration1
{
7)void show(){
1) public static void main(String args[])
8)System.out.println("Super class cal :
{
"+cal);
2)Secondary s = new Secondary(15,22);
}
6)s.show();
}
9)s.display();
class Secondary extends Primary{
}
int cal; //declaration2
}
3)Secondary(int x,int y) { //statement1
First use of the ‘super’ keyword
4)cal = x; //call=15
//statement2
Output:
5)super.cal = y;//cal=22 //statement3
Super class cal : 22
}
Sub class cal : 15
10)void display()
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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.
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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 A() {
}
public A(double d) {
// some statements
}
public A() {
super();
}
is equivalent to
is equivalent to
public A(double d) {
super();
// some statements
}
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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.
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Calling super class constructor
void display(){
//Use of super to access super class
System.out.println("Sub class cal : "+cal);
constructor
}
class Primary{
}
int cal; //declaration1
class SuperUse2{
Primary(int a){
public static void main(String args[]){
cal = a;
Secondary s = new Secondary(15,22);
}
s.show();
void show(){
s.display();
System.out.println("Super class cal : "+cal); }
}
}
}
Use of ‘super’ to access the super class
class Secondary extends Primary{
constructor
int cal; //declaration2
Output:
Secondary(int x,int y) //statement1
{
Super class cal : 22
super(y); //statement2
Sub class cal : 15
cal = x; //statement3
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}
Creating multilevel hierarchy
// Creating multilevel hierarchy
class White{
int num;
7)White(int x)//x=y=z=96 //statement1
{
8)num = x;//num=96
}
}
class Red extends White{
5)Red(int y)//Y=Z=96
{
6)super(y); }
//statement2
10)void print(){
11)System.out.println("Value of num:
"+num);
}
}
class Magenta extends Red{
int maggy;
3)Magenta(int z){//Z=96
4)super(z);
//statement3 maggy = z;
}
}
class MultiLevel
{
1)public static void main(String args[]){
2)Magenta m = new Magenta(96);
//statement4
9)m.print();
}
Creation of multilevel hierarchy
Output:
Value of num: 96
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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
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Calling Superclass Methods
You could rewrite the printCircle() method in the Circle class as
follows:
public void printCircle() {
System.out.println("The circle is created " +
super.getDateCreated() + " and the radius is " + radius);
}
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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.
//Method Overriding
class Maths
{
int var1, var2, var3;
Maths(int x, int y)
{
var1 = x;
var2 = y;}
void calculate()//statement1
{
var3 = var1 + var2;
System.out.println("Addition
:” +var3);}
}
class Arithmetic extends
Maths{
Arithmetic(int x,int y){
super(x,y);}
void calculate()//stmt2
{
var3 = var1 - var2;
System.out.println("Subtrac
tion : "+var3);}
}
class OverRiding{
public static void
main(String args[]){
Arithmetic a = new
Arithmetic(30,18);
a.calculate(); //statement3
}
} Output: Subtraction : 12
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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.
•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.
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Overriding vs. Overloading
public class Test {
public static void main(String[] args) {
A a = new A();
a.p(10);
}
}
public class Test {
public static void main(String[] args) {
A a = new A();
a.p(10);
}
}
class B {
public void p(int i) {
}
}
class B {
public void p(int i) {
}
}
class A extends B {
// This method overrides the method in B
public void p(int i) {
System.out.println(i);
}
}
class A extends B {
// This method overloads the method in B
public void p(double i) {
System.out.println(i);
}
}
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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 {
...
}
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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.
Loan loan = new Loan();
System.out.println(loan.toString());
The code displays something like Loan@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.
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Polymorphism, Dynamic Binding and Generic Programming
public class PolymorphismDemo {
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());
}
}
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.
class GraduateStudent extends Student {
}
class Student extends Person {
public String toString() {
return "Student";
}
}
class Person extends Object {
public String toString() {
return "Person";
}
}
PolymorphismDemo
Run
When the method m(Object x) is executed, the
argument x’s toString method is invoked. x
may be an instance of GraduateStudent,
Student, Person, or Object. Classes
GraduateStudent, Student, Person, and Object
have their own implementation of the toString
method. Which implementation is used will be
determined dynamically by the Java Virtual
Machine at runtime. This capability is known
as dynamic binding.
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// Example for Polymorphism
class A{
void f() {
System.out.println("A.f()"); }
}
class B extends A {
void g() {
System.out.println("B.g()"); }
}
public class Poly{
public static void main(String[] args){
A a = new B();
a.f();
a.g();
}
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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
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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.
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dynamic binding or method overriding
class Animal {
}
public void makeNoise()
public class Demo
{
{
System.out.println("Some sound");
public static void main(String[] args)
{
}
Animal a1 = new Cat();
}
a1.makeNoise(); //Prints Meowoo
class Dog extends Animal{
public void makeNoise()
Animal a2 = new Dog();
{
a2.makeNoise(); //Prints Bark
System.out.println("Bark");
}
}
}
}
class Cat extends Animal{
public void makeNoise()
{
System.out.println("Meawoo");
}
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Generic Programming
public class PolymorphismDemo {
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";
}
}
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.
class Person extends Object {
public String toString() {
return "Person";
}
}
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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.
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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
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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;
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The instanceof Operator
Use the instanceof operator to test whether an object is an instance of
a class:
Object myObject = new Circle();
... // Some lines of code
/** Perform casting if myObject is an instance of
Circle */
if (myObject instanceof Circle) {
System.out.println("The circle diameter is " +
((Circle)myObject).getDiameter());
...
}
class Simple{
public static void main(String args[]){
Simple s=new Simple();
System.out.println(s instanceof Simple);//true
}
}
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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.
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Example: Demonstrating
Polymorphism and Casting
This example creates two geometric objects: a
circle, and a rectangle, invokes the
displayGeometricObject method to display the
objects. The displayGeometricObject displays
the area and diameter if the object is a circle, and
displays area if the object is a rectangle.
TestPolymorphismCasting
Run
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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
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 ArrayList 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 ArrayList class that can
be used to store an unlimited number of objects.
java.util.ArrayList
+ArrayList()
Creates an empty list.
+add(o: Object) : void
Appends a new element o at the end of this list.
+add(index: int, o: Object) : void
Adds a new element o at the specified index in this list.
+clear(): void
Removes all the elements from this list.
+contains(o: Object): boolean
Returns true if this list contains the element o.
+get(index: int) : Object
Returns the element from this list at the specified index.
+indexOf(o: Object) : int
Returns the index of the first matching element in this list.
+isEmpty(): boolean
Returns true if this list contains no elements.
+lastIndexOf(o: Object) : int
Returns the index of the last matching element in this list.
+remove(o: Object): boolean
Removes the element o from this list.
+size(): int
Returns the number of elements in this list.
+remove(index: int) : Object
Removes the element at the specified index.
+set(index: int, o: Object) : Object
Sets the element at the specified index.
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The MyStack Classes
Data1
Data2
Data3
Data2
Data1
Data1
Data3
Data2
Data2
Data1
Data3
Data2
Data1
Data1
Data1
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The MyStack Classes
A stack to hold objects.
MyStack
MyStack
-list: ArrayList
A list to store elements.
+isEmpty(): boolean
Returns true if this stack is empty.
+getSize(): int
Returns the number of elements in this stack.
+peek(): Object
Returns the top element in this stack.
+pop(): Object
Returns and removes the top element in this stack.
+push(o: Object): Object Adds a new element to the top of this stack.
+search(o: Object): int
Returns the position of the first element in the stack from
the top that matches the specified element.
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The protected Modifier
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.
 The
 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();
}
cannot invoke o.m();
}
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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.
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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.
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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.
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