test.java

// To use the sqrt function
import java.lang.Math;

interface Polygon {
  void getArea();

  // calculate the perimeter of a Polygon
  default void getPerimeter(int... sides) {
    int perimeter = 0;
    for (int side : sides) {
      perimeter += side;
    }

    System.out.println("Perimeter: " + perimeter);
  }
}

class Triangle implements Polygon {
  private int a, b, c;
  private double s, area;

  // initializing sides of a triangle
  Triangle(int a, int b, int c) {
    this.a = a;
    this.b = b;
    this.c = c;
    s = 0;
  }

  // calculate the area of a triangle
  public void getArea() {
    s = (double) (a + b + c) / 2;
    area = Math.sqrt(s * (s - a) * (s - b) * (s - c));
    System.out.println("Area: " + area);
  }
}

class Main {
  public static void main(String[] args) {
    Triangle t1 = new Triangle(2, 3, 4);

    // calls the method of the Triangle class
    t1.getArea();

    // calls the method of Polygon
    t1.getPerimeter(2, 3, 4);
  }
}

interface Shape {

  // implicitly public, static and final
  public String LABLE = "Shape";

  // interface methods are implicitly abstract and public
  void draw();

  double getArea();
}