Chapter ii(oop)

Object-Oriented Programming and Java
Java Basic

Introducing Classes
• A template for multiple objects with similar features. Classes embody
all the features of a particular set of objects.
• A class is a reference type, which means that a variable of the class
type can reference an instance of the class.
• An object represents an entity in the real world that can be distinctly
identified.
• object is an instance(Creating) of a class(Data member +Method
member).
• A class library is a set of classes.
• Every Class is made up of two components: attributes and behavior.
– Attributes:state of an object (also known as its properties or attributes)
individual things differentiate one object from another and determine
appearance, state, data field(Table): object’s data fields, or other
qualities of that object.
– Behavior: (also known as its actions) what instances of that class do
when their internal state changes.
• Methods are functions defined inside classes that operate on instances of those
classes.

• An object is an instance of a class. Default value= null
• You can create many instances of a class. Creating an instance is referred to as
instantiation.
• Box mybox; // declare reference to object//a variable of the class type can
reference an instance of the class
• mybox = new Box(); // allocate a Box object//assigns its reference to Box
• The dot operator links the name of the object with the name of an instance
variable.
• dot operator to access both the instance variables and the methods within an
object.
• references a data field in the object.
• invokes a method on the object

General form of class
Public class classname {
type instance-variable1;
type instance-variable2;
// ...
type instance-variableN;
type methodname1(parameter-list) {
// body of method
}
type methodname2(parameter-list) {
// body of method
}
// ...
type methodnameN(parameter-list) {
// body of method
}
}

Circle:
radius: double
Circle(newRadius: double)
getArea(): double
TV:
channel: int
volumeLevel: int
on: boolean
+turnOn(): void
+turnOff(): void
+setChannel(newChannel: int): void
+setVolume(newVolumeLevel: int): void
+channelUp(): void
+channelDown(): void
+volumeUp(): void
+volumeDown(): void

public class Pet {
public int age;
float weight;
float height;
String color;
public void sleep(){
System.out.println(
"Good night, see you tomorrow");
}
public void eat(){
System.out.println(
"I’m so hungry…let me have a snack like nachos!");
}
public String say(String aWord){
String petResponse = "OK!! OK!! " +aWord;
return petResponse;
}
}

public class Test {
String make;
String color;
boolean engineState;
void startEngine()
{
if (engineState == true)
System.out.println("The engine is already on.");
else
{
engineState = true;
System.out.println("The engine is now on.");
}
}
void showAtts()
{
System.out.println("This motorcycle is a " + color + " " + make);
if (engineState == true)
System.out.println("The engine is on.");
else System.out.println("The engine is off.");
}

public static void main(String[] args)
{
Test m = new Test();
m.make = "Yamaha RZ350";
m.color = "yellow";
System.out.println("Calling showAtts...");
m.showAtts();
System.out.println("--------");
System.out.println("Starting engine...");
m.startEngine();
System.out.println("Calling showAtts...");
m.showAtts();
System.out.println("Starting engine...");
m.startEngine();
}
}

class Box {
double width;
double height;
double depth;
}
class TestOOP1 {
public static void main(String args[])
{
Box mybox = new Box();
double vol;
mybox.width = 10;
mybox.height = 20;
mybox.depth = 15;
vol = mybox.width * mybox.height * mybox.depth;
System.out.println("Volume is " + vol);
Box box2=new Box();
}
}

Method
type name(parameter-list) {
// body of method
}

class Box {
double width;
double height;
double depth;
// compute and return volume
double volume() {
return width * height * depth;
}
// sets dimensions of box
void setDim(double w, double h, double d) {
width = w;
height = h;
depth = d;
}}
class BoxDemo5 {
public static void main(String args[]) {
Box mybox1 = new Box();
double vol;
// initialize each box
mybox1.setDim(10, 20, 15);
mybox2.setDim(3, 6, 9);
// get volume of first box
vol = mybox1.volume();
System.out.println("Volume is " + vol);
// get volume of second box
System.out.println("Volume is " + vol);}}

• c1 = c2, c1 points to the same object
• referenced by c2. The object previously referenced by c1 is no longer
useful and therefore is now known as garbage. Garbage occupies
memory space.
• object is no longer needed, you can explicitly assign null to a
reference variable for the object.

Garbage collection
• allocated by using the new operator
• Deallocation is called garbage collection.
objectName=null;
– The finalize( ) Method
protected void finalize( )
{
// finalization code here
}

Constructors• methods of a special type, known as constructors
• A constructor must have the same name as the class itself.
• It can be tedious to initialize all of the variables in a class each time an instance is created by
Method set.
• Constructors are invoked using the new operator when an object is created. Constructors
play the role of initializing objects.
• A constructor initializes an object immediately upon creation.
• look a little strange because they have no return type, not even void.
• Constructors are designed to perform initializing actions, such as initializing the data fields of
objects.
Box() {
System.out.println("Constructing Box");
width = 10;
height = 10;
depth = 10;
}
Or
Box(double w, double h, double d) {
width = w;
height = h;
depth = d;
}
Box mybox1 = new Box(2,3,1);
Box mybox2 = new Box(2,4,1);

Overloading Constructors
class Box {
double width;
double height;
double depth;
width = w;
height = h;
depth = d;
}
Box() {
width = -1; // use -1 to indicate
height = -1; // an uninitialized
depth = -1; // box
}
Box(double len) {
width = height = depth = len;
}
double volume() {
}}
class OverloadCons {
// create boxes using the various constructors
Box mybox1 = new Box(10, 20, 15);
Box mycube = new Box(7);
double vol;
// get volume of first box
System.out.println("Volume of mybox1 is " +
vol);
// get volume of second box
vol);
// get volume of cube
vol = mycube.volume();
System.out.println("Volume of mycube is " +
vol);
}
}

The this Keyword
• be used inside any method to refer to the current object.
this.width = w;
this.height = h;
this.depth = d;
}
// Use this to resolve name-space collisions.
Box(double width, double height, double depth) {
this.width = width;
this.height = height;
this.depth = depth;
}

Overloading Methods
• two or more methods within the same class
that share the same name, as long as their
parameter declarations are different.

class OverloadDemo {
void test() {
System.out.println("No parameters");
}
void test(int a) {
System.out.println("a: " + a);
}
void test(int a, int b) {
System.out.println("a and b: " + a + " " + b);
}
double test(double a) {
System.out.println("double a: " + a);
return a*a;
}
}
class Overload {
OverloadDemo ob = new OverloadDemo();
double result;
ob.test();
ob.test(10);
ob.test(10, 20);
result = ob.test(123.25);
System.out.println("Result of ob.test(123.25): " + result);
}}

Using Objects as Parameters
• Like passing an array, passing an object is actually passing the reference of the object.
class Test {
int a, b;
Test(int i, int j) {
a = i;
b = j;
}
// return true if o is equal to the invoking object
boolean equals(Test o) {
if(o.a == a && o.b == b) return true;
else return false;
}
}
class PassOb {
Test ob1 = new Test(100, 22);
Test ob2 = new Test(100, 22);
Test ob3 = new Test(-1, -1);
System.out.println("ob1 == ob2: " + ob1.equals(ob2));
System.out.println("ob1 == ob3: " + ob1.equals(ob3));
}
}

class Box {
double width;
double height;
double depth;
Box(Box ob) { // pass object to constructor
width = ob.width;
height = ob.height;
depth = ob.depth;
}
// constructor used when all dimensions specified
width = w;
height = h;
depth = d;
}
// constructor used when no dimensions specified
Box() {
depth = -1; // box
}
// constructor used when cube is created
Box(double len) {
}
double volume() {
}
}
class OverloadCons2 {
Box mybox1 = new Box(10, 20, 15);
Box mycube = new Box(7);
Box myclone = new Box(mybox1);
double vol;
vol);
vol);
vol = mycube.volume();
System.out.println("Volume of cube is " + vol);
vol = myclone.volume();
System.out.println("Volume of clone is " + vol);
}}

Returning Objectsclass Test {
int a;
Test(int i) {
a = i;
}
Test incrByTen() {
Test temp = new Test(a+10);
return temp;
}
}
class RetOb {
Test ob1 = new Test(2);
Test ob2;
ob2 = ob1.incrByTen();
System.out.println("ob1.a: " + ob1.a);
System.out.println("ob2.a: " + ob2.a);
ob2 = ob2.incrByTen();
System.out.println("ob2.a after second increase: "
+ ob2.a);}}

Array of object
• declares and creates an array of ten Circle objects:
– Circle[] circleArray = new Circle[10];
• To initialize the circleArray, you can use a for loop like this one:
for (int i = 0; i < circleArray.length; i++) {
circleArray[i] = new Circle();
}
• An array of objects is actually an array of reference
variables
• invoking circleArray[1].getArea() involves two levels of
referencing, as shown in Figure 8.18. circleArray references
the entire array. circleArray[1] references a Circle object.
• When an array of objects is created using the new
operator, each element in the array is a reference variable
with a default value of null.

Static Variables, Constants, and Methods
• Static variables store values for the variables in a
common memory location.
• Static methods can be called without creating an
instance of the class.

Introducing Access Control
• public specifier, then that member can be accessed by any other code.
This is not good for 2 reason
– 1. arbitrary value
– 2. class becomes difficult to maintain and vulnerable to bugs.
• private, then that member can only be accessed by other members of its
class. To prevent direct modifications of data fields, you should declare the
data fields private,using the private modifier. This is known as data field
encapsulation.
• protected applies only when inheritance is involved.
• Colloquially, a get method is referred to as a getter (or accessor), and a set
method is referred to as a setter (or mutator).

Inheritance – a Fish is Also a Pet
• Object-oriented programming allows you to derive new
classes from existing classes called inheritance.
• best way to design these classes so to avoid redundancy
and make the system easy to comprehend and easy to
maintain?
• every person inherits some features from his or her
parents.
• behavior and attributes that are shared by many pets.
• pets are different - dogs bark, fish swim and do not make
sounds, parakeets talk better than dogs. But all of them
eat, sleep, have weight and height.
• Fish is a subclass of the class Pet or Pet as a template for
creating a class Fish.

class GeometricObject1 {
public String color = "white";
private boolean filled;
private java.util.Date dateCreated;
public GeometricObject1() {
dateCreated = new java.util.Date();}
public GeometricObject1(String Color, boolean filled)
{dateCreated = new java.util.Date();
this.color = color;
this.filled = filled;}
public String getColor() {
return color;}
public void setColor(String color) {
this.color = color;}
public boolean isFilled() {
return filled;}
public void setFilled(boolean filled) {
this.filled = filled;}
public java.util.Date getDateCreated() {
return dateCreated; }
public String toString() {
return "created on " + dateCreated + "ncolor: " +
color + " and filled: " + filled;}}
class Circle4 extends GeometricObject1 {
private double radius;
public Circle4() {}
public Circle4(double radius) {
this.radius = radius;
color=“red”;}
public Circle4(double radius, String color,
boolean filled) {
this.radius = radius;
setColor(color);
setFilled(filled); }
public double getRadius() {
return radius;}
public void setRadius(double radius) {
this.radius = radius; }
public double getArea() {
return radius * radius * Math.PI;}
public double getDiameter() {
return 2 * radius; }
public double getPerimeter() {
return 2 * radius * Math.PI; }
public void printCircle() {
System.out.println("The circle is created " + " and the
radius is " + radius); }
}

class Rectangle1 extends GeometricObject1 {
private double width;
private double height;
public Rectangle1() {}
public Rectangle1(double width, double height) {
this.width = width;
this.height = height;}
public Rectangle1(double width, double height,
String color,
boolean filled) {
this.width = width;
this.height = height;
setColor(color);
setFilled(filled);}
public double getWidth() {
return width;}
public void setWidth(double width) {
this.width = width; }
public double getHeight() {
return height; }
public void setHeight(double height) {
this.height = height; }
public double getArea() {
return width * height; }
public double getPerimeter() {
return 2 * (width + height);
} }
public class Test3 {
public static void main(String[] args) {
Circle4 circle = new Circle4(1);
System.out.println("A circle "+ circle.toString());
System.out.println("The radius is "+ circle.getRadius() );
System.out.println("The area is "+ circle.getArea());
System.out.println("The diameter is "+
circle.getDiameter());
Rectangle1 rectangle = new Rectangle1(2, 4);
System.out.println("nA rectangle "+
rectangle.toString());
System.out.println("The area is "+
rectangle.getArea());
System.out.println("The perimeter is " +
rectangle.getPerimeter() );
}
}

Creating a Multilevel Hierarchy
class Box {
private double width;
private double height;
private double depth;
// construct clone of an object
Box(Box ob) { // pass object to constructor
width = ob.width;
height = ob.height;
depth = ob.depth;
}
// constructor used when all dimensions specified
width = w;
height = h;
depth = d;
}
// constructor used when no dimensions specified
Box() {
depth = -1; // box
}
Box(double len) {
}
double volume() {
}
}
// Add weight.
class BoxWeight extends Box {
double weight; // weight of box
BoxWeight(BoxWeight ob) { // pass object to
constructor
super(ob);
weight = ob.weight;
}
// constructor when all parameters are specified
BoxWeight(double w, double h, double d, double m)
{
super(w, h, d); // call superclass constructor
weight = m;
}
// default constructor
BoxWeight() {
super();
weight = -1;
}
BoxWeight(double len, double m) {
super(len);
weight = m;
}
}

class Shipment extends BoxWeight {
double cost;
Shipment(Shipment ob) { // pass object to constructor
super(ob);
cost = ob.cost;
}
// constructor when all parameters are specified
Shipment(double w, double h, double d,
double m, double c) {
super(w, h, d, m); // call superclass constructor
cost = c;
}
// default constructor
Shipment() {
super();
cost = -1;
}
Shipment(double len, double m, double c) {
super(len, m);
cost = c;
}
}
class DemoShipment {
Shipment shipment1 =
new Shipment(10, 20, 15, 10, 3.41);
Shipment shipment2 =
new Shipment(2, 3, 4, 0.76, 1.28);
double vol;
vol = shipment1.volume();
System.out.println("Volume of shipment1 is " +
vol);
System.out.println("Weight of shipment1 is "
+ shipment1.weight);
System.out.println("Shipping cost: $" +
shipment1.cost);
System.out.println();
vol = shipment2.volume();
System.out.println("Volume of shipment2 is " +
vol);
System.out.println("Weight of shipment2 is "
+ shipment2.weight);
System.out.println("Shipping cost: $" +
shipment2.cost);
}
}

• is the process by which one object acquires the properties of
another object.

class A {
int i, j;
void showij() {
System.out.println("i and j: " + i + " " + j);
}
}
// Create a subclass by extending class A.
class B extends A {
int k;
void showk() {
System.out.println("k: " + k);
}
void sum() {
System.out.println("i+j+k: " + (i+j+k));
}
}
class SimpleInheritance {
A superOb = new A();
B subOb = new B();
// The superclass may be used by itself.
superOb.i = 10;
superOb.j = 20;
System.out.println("Contents of superOb: ");
superOb.showij();
/* The subclass has access to all public members
of
its superclass. */
subOb.i = 7;
subOb.j = 8;
subOb.k = 9;
System.out.println("Contents of subOb: ");
subOb.showij();
subOb.showk();
System.out.println("Sum of i, j and k in subOb:");
subOb.sum();
}
}

Using the super Keyword
• A subclass inherits accessible data fields and methods from its
superclass.
• The keyword super refers to the superclass of the class in which
super appears. It can be used in two ways:
– To call a superclass constructor.
• super(), or super(parameters);
– To call a superclass method.
• super.method(parameters);
• public void printCircle() {
System.out.println("The circle is created " +
super.getDateCreated() + " and the radius is " + radius);}
• It is not necessary to put super before getDateCreated() in this case,
however, because getDateCreated is a method in the
GeometricObject class and is inherited by the Circle class.

Method Overriding
• Sometimes it is necessary for the subclass to modify the
implementation of a method defined in the superclass.
• hierarchy, when method in subclass has the same name and type
signature as a method in its superclass, then the method in the
subclass is said to override the method in the superclass.
• means a subclass method overriding a super class method.
• Benefit: avoid using method in class super
• To override a method, the method must be defined in the subclass
using the same signature and the same return type.

// Method overriding.
class A {
int i, j;
A(int a, int b) {
i = a;
j = b; }
// display i and j
void show() {
System.out.println("i and j: " + i + " " + j);
} }
class B extends A {
int k;
B(int a, int b, int c) {
super(a, b);
k = c; }
// display k – this overrides show() in A
void show() {
System.out.println("k: " + k); }
}
class Override {
B subOb = new B(1, 2, 3);
A subOba;
subOb.show(); // this calls show() in B } }

Overriding vs. Overloading
• Overloading means to define multiple methods with the same name
but different signatures.
• Overriding means to provide a new implementation for a method in
the subclass. The method is already defined in the superclass.
• To override a method, the method must be defined in the subclass
using the same signature and the same return type.

Using final to Prevent Overriding
class A {
final void meth() {
System.out.println("This is a final method.");
}
}
class B extends A {
void meth() { // ERROR! Can't override.
System.out.println("Illegal!");
}
}

Using final to Prevent Inheritance
final class A {
// ...
}
// The following class is illegal.
class B extends A { // ERROR! Can't subclass A
// ...
}

Polymorphism (from a Greek word meaning “many forms”)
• three pillars of object-oriented programming are encapsulation,
inheritance, and polymorphism.
• A class defines a type. A type defined by a subclass is called a subtype
and a type defined by its superclass is called a supertype.
• SubClass is a subtype of SuperClass and SuperClass is a supertype for
SubClass.
• every circle is a geometric object, but not every geometric object is a
circle. Therefore, you can always pass an instance of a subclass to a
parameter of its superclass type
• In simple terms, polymorphism means that a variable of a supertype
can refer to a subtype object.

Method displayObject takes a parameter of the GeometricObject type. You
can invoke displayObject by passing any instance of GeometricObject (e.g., new Circle4(
1, "red", false) and new Rectangle1(1, 1, "black", false). An
object of a subclass can be used wherever its superclass object is used. This is commonly
known as polymorphism (from a Greek word meaning “many forms”). In simple terms,
polymorphism means that a variable of a supertype can refer to a subtype object.

Using Abstract Classes
• Any subclass of an abstract class must either implement all of the abstract methods
in the superclass, or be itself declared abstract.
– abstract type name(parameter-list);
• Not possible to instantiate an abstract class
• cannot be used to instantiate(represent) objects, used to create object references.
• classes become more specific and concrete with each new subclass
• Class design should ensure that a superclass contains common features of its
subclasses.
• Abstract method define them only in each subclass.
• abstract class cannot be instantiated using the new operator, but you can still
define its constructors, which are invoked in the constructors of its subclasses.
• A class that contains abstract methods must be abstract. However, it is possible to
define an abstract class that contains no abstract methods

// A Simple demonstration of abstract.
abstract class A {
abstract void callme();
// concrete methods are still allowed in abstract classes
void callmetoo()
{
System.out.println("This is a concrete method.");
}
}
class B extends A {
void callme() {
System.out.println("B's implementation of callme.");
}
}
class AbstractDemo {
//A a=new A(); cannot be created object because of abstract class
B b = new B();
b.callme();
b.callmetoo();
}
}
A, it is not possible to instantiate an abstract class.

// Using abstract methods and classes.
abstract class Figure {
double dim1;
double dim2;
Figure(double a, double b) {
dim1 = a;
dim2 = b;
}
// area is now an abstract method
abstract double area();
}
class Rectangle extends Figure {
Rectangle(double a, double b) {
super(a, b);
}
// override area for rectangle
double area() {
System.out.println("Inside
Area for Rectangle.");
return dim1 * dim2;
}
}
class Triangle extends Figure {
Triangle(double a, double b) {
super(a, b);
}
// override area for right triangle
double area() {
System.out.println("Inside Area for Triangle.");
return dim1 * dim2 / 2;
}
}
class AbstractAreas {
// Figure f = new Figure(10, 10); // illegal now
Rectangle r = new Rectangle(9, 5);
Triangle t = new Triangle(10, 8);
Figure figref; // this is OK, no object is created
figref = r;
System.out.println("Area is " + figref.area());
figref = t;
System.out.println("Area is " + figref.area());
}
}

Packages
• a mechanism for partitioning the class name space
into more manageable chunks.
• unique name had to be used for each class to avoid
name collisions.
• Java uses file system directories to store packages
– package pkg1[.pkg2[.pkg3]];
– Ex: package java.awt.image; (be stored in javaawtimage
in a Windows environment)

D:MyPack => D:MyPackjavac AccountBalance.java=> D:java MyPack.AccountBalance.java
// A simple package
package MyPack;
class Balance {
String name;
double bal;
Balance(String n, double b) {
name = n;
bal = b;
}
void show() {
if(bal<0)
System.out.print("--> ");
System.out.println(name + ": $" + bal);
}
}
class AccountBalance {
Balance current[] = new Balance[3];
current[0] = new Balance("K. J. Fielding", 123.23);
current[1] = new Balance("Will Tell", 157.02);
current[2] = new Balance("Tom Jackson", -12.33);
for(int i=0; i<3; i++) current[i].show();
}
}

D:worldHelloWorld
package world;
public class HelloWorld
{
public static void main(String[] args)
{
System.out.println("Hello World");
}
}
D:worldjavac HelloWorld
D:java world.HelloWorld

Importing Packages
• import pkg1[.pkg2].(classname|*);
– import java.util.*;
class MyDate extends Date {
}
– class MyDate extends java.util.Date { }

package MyPack;
public class Balance {
String name;
double bal;
public Balance(String n, double b) {
name = n;
bal = b;
}
public void show() {
if(bal<0)
System.out.print("--> ");
System.out.println(name + ": $" + bal);
}}
-------------------------------------------
import MyPack.*;
class TestBalance {
/* Because Balance is public, you may use Balance
class and call its constructor. */
Balance test = new Balance("J. J. Jaspers", 99.88);
test.show(); // you may also call show()
}
}

Interface
• specify what a class must do, but not how it does it.
• similar to classes, but they lack instance variables, and their methods are declared without any body.
access interface name {
return-type method-name1(parameter-list);
return-type method-name2(parameter-list);
type final-varname1 = value;
type final-varname2 = value;
// ...
return-type method-nameN(parameter-list);
type final-varnameN = value;
}

interface Callback {
void callback(int param);
}
class Client implements Callback {
// Implement Callback's interface
public void callback(int p) {
System.out.println("callback called with " + p);
}
}
class TestIface {
Callback c = new Client();
c.callback(42);
}
}
Note:
// Another implementation of Callback.
class AnotherClient implements Callback {
// Implement Callback's interface
public void callback(int p) {
System.out.println("Another version of
callback");
System.out.println("p squared is " + (p*p));
}
}
class TestIface2 {
Callback c = new Client();
AnotherClient ob = new AnotherClient();
c.callback(42);
c = ob; // c now refers to AnotherClient
object
c.callback(42);
}
}
callback called with 42
Another version of callback
p squared is 1764

Chapter ii(oop)

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Chapter ii(oop)

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