Introduction

In this demo I have used NetBeans IDE 8.2 for debugging purpose. But you can use any java programming language compiler as per your availability..

import java.util.Scanner;
public class Javaexcercise {
  public static void main(String[] args)
    {
        Scanner in = new Scanner(System.in);
         System.out.print("Input Side-1: ");
         double side1 = in.nextDouble();
         System.out.print("Input Side-2: ");
         double side2 = in.nextDouble();
         System.out.print("Input Side-3: ");
         double side3 = in.nextDouble();
         System.out.println( isValid(side1, side2,side3) ?
                "The area of the triangle is " + areaoftriangle(side1, side2, side3) : "Invalid triangle" );
    }
 
  public static boolean isValid(double side1, double side2, double side3) {
        if( side1 + side2 > side3 &&
                side2 + side3 > side1 &&
                side1 + side3 > side2) return true;
        else return false;
    }
   public static double areaoftriangle(double side1, double side2, double side3) {
        double area = 0;
        double s = (side1 + side2 + side3)/2;
        area = Math.sqrt(s*(s - side1)*(s - side2)*(s - side3));
        return area;
    }
}

Result