Introductory Core Java Programs

1. Program to Calculate Area of a Circle (radius is taken as user input).

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class CalculateCircleAreaExample {

        public static void main(String[] args) {
               
                int radius = 0;
                System.out.println("Please enter radius of a circle");
               
                try
                {
                        //get the radius from console
                        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
                        radius = Integer.parseInt(br.readLine());
                }
                //if invalid value was entered
                catch(NumberFormatException ne)
                {
                        System.out.println("Invalid radius value" + ne);
                        System.exit(0);
                }
                catch(IOException ioe)
                {
                        System.out.println("IO Error :" + ioe);
                        System.exit(0);
                }
               
                /*
                 * Area of a circle is
                 * pi * r * r
                 * where r is a radius of a circle.
                 */
               
                //NOTE : use Math.PI constant to get value of pi
                double area = Math.PI * radius * radius;
               
                System.out.println("Area of a circle is " + area);
        }
}
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Output of Calculate Circle Area using Java Example would be

Please enter radius of a circle
19
Area of a circle is 1134.1149479459152