1. Lecture 1
Object Oriented Programming
Object Oriented Programming
Eastern University, Dhaka
Md. Raihan Kibria

2. Why object oriented programming
Consider the following program written in C:

3. #include <stdio.h>
#include <math.h>
#include <stdlib.h>
struct cgpa{
int grades[3];
}ca;
float getCgpa1(struct cgpa cgpa){
float sum = 0;
int i = 0;
for (i=0;i<3; i++){
sum += cgpa.grades[i];
}
return sum/3;
}

4. float getCgpa2(struct cgpa cgpa){
float sum = 0;
int i = 0;
for (i=0;i<3; i++){
sum += cgpa.grades[i];
}
return roundf(sum/3);
}

5. float getCgpa3(struct cgpa cgpa){
float sum = 0;
int i = 0;
for (i=0;i<3; i++){
sum += cgpa.grades[i];
}
return abs(sum/3);
}
int main(){
printf ("%s", "a message to startn");
ca.grades[0] = 3;
ca.grades[1] = 5;
ca.grades[2] = 6;
printf("%fn", getCgpa1(ca));
printf("%fn", getCgpa2(ca));
printf("%fn", getCgpa3(ca));
}

6. OOP goals
 Maximize code re-use
 Minimize re-coding

7. Same program in java
public class Grade {
public static void main(String[] args) {
System.out.println(new Grade1().getCgpa());
System.out.println(new Grade2().getCgpa());
System.out.println(new Grade3().getCgpa());
}
}
class Grade1{
int[] grades = {3,5,6};
float sum;
protected void calculateSum(){
for (int i=0; i<grades.length; i++)
sum += grades[i];
}
public float getCgpa(){
calculateSum();
return sum / grades.length;
}
}
class Grade2 extends Grade1{
public float getCgpa(){
calculateSum();
return Math.round(sum / grades.length);
}
}
class Grade3 extends Grade1{
public float getCgpa(){
calculateSum();
return (float)Math.floor(sum / grades.length );
}
}

8. How to start programming in OOP
 Class
A class is a template
//a student
public class ClassDemo {
String code;
String name;
}

9. Entry point
public static void main(String[] args) {}
e.g.
//a student
public class ClassDemo {
String code;
String name;
public static void main(String[] args){
ClassDemo demo = new ClassDemo();
demo.code = "1221232323";
demo.name = "Raihan Kibria";
System.out.println(demo.code);
System.out.println(demo.name);
}
}

10. How to compile
Install jdk
Compile:
javac ClassDemo.java
How to run:
java ClassDemo