OOP: 1 Understand the Difference Between Procedural and Object-Oriented Programming

One of the four basic principles of object-oriented programming (OOP) is encapsulation. Encapsulation is a mechanism of hiding data implementation by restricting access to public methods.

This topic was my first task in the OOP lesson. For example, this is the procedural version that I wrote in C.

#include <stdio.h>

#define PI 3.14159265359

int main(){

        double r;

        scanf("%lf\n", &r);

        /*For example, I can add to the main

        r = 3.0;

        to directly reference variable r. *\

        printf("Radius: %.2f\n", r);

        printf("Circumference: %.2f\n", 2 * PI * r);

        printf("Area: %.2f\n", PI * r * r);

        return 0;

}

The solution is to encapsulate the method using Java class and access modifier. 

For example, this is the circle class. 

package id.ac.its.fortunela.ling;

public class Lingkaran {

        private double radius;

        private double pi = 3.14159265359;

        public void setRadius(int radius) {

                this.radius = radius;

        }

        public double getRadius() {

                return radius;

        }

        public double getKeliling() {

                return 2 * pi * getRadius();

        }

        public double getLuas() {

                return pi * pow(getRadius(),2);

        }

}

And this is the main class.

package id.ac.its.fortunela.ling;

import java.util.Scanner;

import java.text.DecimalFormat;

public class MainApp {

        public static void main(String[] args) {

                /*As you can see, it is not possible to directly reference variable radius. *\

                Lingkaran li = new Lingkaran();

                Scanner in = new Scanner(System.in);   

                double d = in.nextDouble();

                /*Using setRadius makes variable radius can't directly referenced*\

                li.setRadius(d);

                in.close();

                DecimalFormat numberFormat = new DecimalFormat("#.00");

                System.out.println("Radius: " + numberFormat.format(li.getRadius()));

                System.out.println("Luas: " + numberFormat.format(li.getLuas()));

                System.out.println("Keliling: " + numberFormat.format(li.getKeliling()));

        }

}

Well, my Java code wasn't looking good when I checked it now. So above is the corrected version. You know, I was confident in OOP before, but now I am anxious that my grades might be... Well, I hope my GPA is more than 3.5 at the least, so I can take some extra credit this term.