Constant E and PI

static double E: base of the natural logarithms.
static double PI: pi.


public class Main {
  public static void main(String[] args) throws Exception {
    System.out.println(Math.E);
    System.out.println(Math.PI);
  }

}

The output:


2.718281828459045
3.141592653589793

Calculate Circle Area using it's radius

 


public class Main {
  public static void main(String[] args) {
    int radius = 5;

    double area = Math.PI * radius * radius;

    System.out.println("Area of a circle is " + area);
  }
}

The output:


Area of a circle is 78.53981633974483

Calculate Circle Perimeter with radius

 
public class Main {
  public static void main(String[] args) {
    int radius = 5;

    double perimeter = 2 * Math.PI * radius;
    System.out.println("Perimeter: " + perimeter);
  }
}


Perimeter: 31.41592653589793