04. Review OOP with Java

04. Review OOP with Java
Oum Saokosal
Master of Engineering in Information Systems, South Korea
855-12-252-752
oum_saokosal@yahoo.com

Agenda
• Array
• Class/Object
• Inheritance
• Abstract Class
• Overridden Methods
• Interface
• Polymorphism

Array
• int[] a = new int[10];
a[0]=10;
a[1]=20;
a[9]=45;
• double[] d = new double[5][10];
d[0][0]=9.9;
d[0][1]=3.14;
a[4][9]=9.8;

Class/Object
public class Aclass {
public Aclass(){//it’s called constructor
Bclass b = new Bclass(); //b is object/instance
b.hello(); //b invokes hello() method
}
public static void main(String[] args){
new Aclass();
}
}
class Bclass {
public void hello(){ //hello() is a method
System.out.print("Hello Everyone");
}
}

Inheritance
in UML Diagram
Child1
Parent
Child2 Child3

Inheritance
public class Child extends Parent{
public Child(){
System.out.println(money);
System.out.println(gold);
System.out.println(rich());
System.out.println(job());
}
public static void main(String[] args){
new Child();
}
}
class Parent{
public double money = 500000;
public double gold = 200;
public boolean rich(){
return true;
}
public String job(){
return "Doctor";
}
}

Abstract class
in UML Diagram
Duck
<<abstract>>
Bird
Swallow

Abstract Class
public abstract class Bird {
public String color;
public abstract boolean isFlyable();
public void sound(){
System.out.println("Default”);
}
}
class Duck extends Bird{
public boolean isFlyable() {
return false;
}
System.out.println("Quack ");
}
}
class Swallow extends Bird{
public boolean isFlyable() {
return true;
}
}
Overridden Method

Interface
public interface Flyable {
public String fly();
}
class Chicken implements Flyable{
@Override
public String fly() {
return "Low and near";
}
}
class Swallow implements Flyable{
@Override
public String fly() {
return "High and far";
}
}Chicken
<<interface>>
Flyable
Swallow

Abstract class vs Interface (Same)
Abstract class
• All subclasses of the
abstract class have to
override all abstract
methods.
• You cannot use new
operator to create an
instance from an abstract
class.
Bird b = new Bird();
• Abstract class can be used
as a type.
Bird b;
Interface
• All classes implemented the
interface have to override
all methods.
• You cannot use new
operator to create an
instance from an interface.
Flyable fly = new Flyable();
• Interface can be used as a
type.
Flyable fly;

Abstract class vs Interface (Different)
Abstract class
• To declare an abstract class,
use abstract keyword.
public abstract class B{
}
• A class can extend only one
abstract class.
class A extends B{
}
• In relationship, we say
A is B.
Interface
• To declare an interface, use
abstract keyword.
public interface B{
}
• A class can implement more
than one interface.
class A implements C, D, E{
}
• In relationship, we
A has C, D, and E.

Polymorphism
What is polymorphism?
• Polymorphism is a mechanism to allow a
single variable to refer to objects from
different classes.

Polymorphism
public class MyPoly {
public static void main(String[] args) {
Bird b; // b is Bird
b = new Duck(); // b refers to Duck
b.sound(); // b invokes Duck’s sound
b = new Chick(); // b refers to Chick
b.sound(); //b invokes Chick’s sound
}
}
abstract class Bird{
public abstract void sound();
}
class Duck extends Bird{
System.out.println("Quack");
}
}
class Chick extends Bird{
System.out.println("Cheep");
}
}

04. Review OOP with Java

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