如何要求用户输入变量？

import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner inp = new Scanner(System.in);
        double H = 1.008;
        double Du = 5.303;
        double Bz = 8.148;
        double Ju = 10.026;
        double Nu = 14.289;
        double M = 14.759;
        double Af = 17.298;
        double Et = 18.77;
        double Z = 16.515;
        double Sl = 20.47;
        double Di = 21.435;
        double Bo = 23.953 ; 
        double Az = 28.302;
        double P = 32.248;
        double Hd = 32.636;
        double Tm = 35.085;
        double R = 36.501;  
        double V = 38.853;
        double Nv = 41.623;
        double A = 42.385;
        double T = 44.466;
        double Ha = 46.224;
        double Jo = 49.014;
        double Fg = 50.835;
        double K = 51.241;
        double Ea = 54.241;
        double B = 57.546;
        double Lu = 61.416;
        double Do = 66.244;
        double Qu = 68.081;
        double En = 70.787;
        double Ka = 72.317;
        double W = 75.462;
        double Of = 79.264;
        double Da = 81.047;
        double Re = 84.941;

        System.out.print("\n What elements");
        String elementletter1 = inp.nextLine();
        double element1 = elementletter1; // error
        System.out.print("\n How many");
        double element1num = inp.nextDouble();
        System.out.print("\n What elements");
        String elementletter2 = inp.nextLine();
        double element2 = elementletter2; // error
        System.out.print("\n How many");
        double element2num = inp.nextDouble();
        System.out.print("what");
        String what = inp.nextLine();

        if (what.equals("moles")) {
            System.out.println("How many grams");
            double grams = inp.nextDouble();
            System.out.println("moles=" + (grams / ((element1 * element1num) + (element2 * element2num))));
        }
        else if (what.equals("grams")) {
            System.out.println("How many moles");
            double moles = inp.nextDouble();
            System.out.println("grams=" + (moles * ((element1 * element1num)   + (element2 * element2num))));
        }
        else if (what.equals("per")) {
            System.out.println("grams per mole =" + (((element1 * element1num) + (element2 * element2num))));
        }
    }
}