The sides of a triangle is constrained by the rule that the sum of any of its two sides is always bigger than the third side. Using this rule, we can find whether 3 sides given for a triangle is valid or not. The following sample Java program checks whether the given 3 sides can form a valid triangle.

import java.util.Scanner;

/**
 * Validates whether the given 3 sides can form a triangle or not.
 * @author jj
 */
public class ValidTriangle {
    
    public static void main(String[] args) {
        
        Scanner sn = new Scanner(System.in);
        System.out.println("Enter length of first side of triangle:");
        double a = sn.nextDouble();
        
        System.out.println("Enter length of second side of triangle:");
        double b = sn.nextDouble();
        
        System.out.println("Enter length of third side of triangle:");
        double c = sn.nextDouble();
        
        ValidTriangle vt   = new ValidTriangle();
        if(vt.isTriangleValid(a, b, c)) {
            System.out.println("Sides entered can form a triangle!");
        }else {
            System.out.println("Sides entered cannot form a triangle!");
        }
        
    }

   
    /**
     * Checks whether the 3 sides can form a triangle
     * The sum of any two sides must be greater than the other side
     * @param a
     * @param b
     * @param c
     * @return 
     */
    private boolean isTriangleValid(double a, double b, double c) {
        if((a+b)>c && (a+c)>b && (b+c)>a) {
            return true;
        }else {
            return false;
        }
    }
}