13.5

//13.5
import java.util.Scanner;
import java.awt.*;

import java.util.*;
import javax.swing.*;
import javax.swing.border.LineBorder;

public class Test
{
    public static void main(String[] args) {
        double s1 = 0,s2 = 0,s3 = 0;
        Scanner input = new Scanner(System.in);
        s1 = input.nextDouble();
        s2 = input.nextDouble();
        s3 = input.nextDouble();
        try {Triangle t1 = new Triangle(s1,s2,s3);}
        catch(IllegalTriangleException ex){
            System.out.println(ex.getMessage());
        }
      }
}

Test.java

public class Triangle {
    private double side1,side2,side3;

    public Triangle(double side1,double side2,double side3)
    throws IllegalTriangleException{
        if((side1+side2>side3)&&(side1+side3>side2)&&(side2+side3>side1))
            System.out.println("valid sides");
        else
            throw new IllegalTriangleException("invalid sides");
    }
}

Triangle.java

public class IllegalTriangleException extends Exception{
    public IllegalTriangleException(String s)
    {
        super(s);
    }
}

IllegalTriangleException.java