How to create a helper method that takes 2 double inputs into a class global array?

I'm new to java and I have a question about an assignment I have. I've written a bunch of methods that are different formulas, like the duration of a storm. The assignment asks me to write two helper methods to get input from the user. One of them is called get_S_Input() and I was able to implement it correctly I think. But the one I'm stuck on is this other helper method called get_2_Invals(). It wants me to prompt the user with my parameter, and read in 2 double values. It wants me to record the values in a class global array of doubles and then exit the method, but I don't know how to do this. I want to put it in the else statement in the method below. Here is my code so far...

import java.lang.Math;
import java.util.Scanner;


public class FunFormulas {
    public void sd(){
        double durationOfStorm = Math.sqrt(((Math.pow(get_S_Input("Enter the diameter of storm in miles:"), 3) / 216)));
        if (durationOfStorm > 0)
            System.out.println("The storm will last: " + durationOfStorm);
    }

    public void sl(){
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter the number of seconds since the lightning strike:");
        double secondsSinceLightning = Double.valueOf(scanner.nextLine());
        double distanceFromLightning = (1100 * secondsSinceLightning);
        System.out.println(distanceFromLightning);
    }

    public void si(){
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter the edge of cube in inches:");
        double edgeOfCubeInInches = Double.valueOf(scanner.nextLine());
        if (edgeOfCubeInInches < 0){
            System.out.println("ERROR: please enter a non-negative number!!!");
        }
        double weightOfCube = (0.33 * (Math.pow(edgeOfCubeInInches, 3)));
        System.out.println(weightOfCube);
    }

    public void dt(){
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter the time in hours:");
        double timeInHours = Double.valueOf(scanner.nextLine());
        if (timeInHours < 0){
            System.out.println("ERROR: please enter a non-negative number!!!");
        }
        System.out.println("Enter the rate of speed in mph:");
        double rateOfSpeed = Double.valueOf(scanner.nextLine());
        if (rateOfSpeed < 0){
            System.out.println("ERROR: please enter a non-negative number!!!");
        }
        double distanceTravelled = (rateOfSpeed * timeInHours);
        System.out.println(distanceTravelled);
    }

    public void sa(){
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter your weight in pounds:");
        double weight = Double.valueOf(scanner.nextLine());
        weight = weight * 0.4536;
        System.out.println("Enter your height in inches:");
        double height = Double.valueOf(scanner.nextLine());
        height = height * 2.54;
        double BSA = ((Math.sqrt(weight * height)) / 60);
        System.out.println(BSA);
    }

    public double get_S_Input(String promptStr){
        //Scanner helper method
        System.out.println(promptStr);
        Scanner scanner = new Scanner(System.in);
        double value = Double.valueOf(scanner.nextLine());
        if (value < 0 ){
            System.out.println("ERROR: please enter a non-negative number!!!");
        }
        return value;
    }

    public void get_2_Invals(String promptStr){
        /*Prompt the user with the promptStr passed in as a parameter
        ii. Read in the first double precision value entered by the user with the Scanner
        iii. Read in the second double precision value entered by the user with the Scanner
        iv. Check to make sure the values entered by the user are non-negative
        a. If either number entered by the user is negative, the method should print out an
            error message, and return to step i. above.
        b. If the number is non-negative (that is greater than or equal to zero) the method
            should record the numbers obtained from the user in a class-global array of
            doubles and exit.*/
        System.out.println(promptStr);
        Scanner scanner = new Scanner(System.in);
        double firstValue = scanner.nextInt();
        double secondValue = scanner.nextInt();
        if (firstValue < 0 || secondValue < 0)
            System.out.println("ERROR: please enter non-negative number!");
        else

    }

    public static void main(String [] args){
        FunFormulas fun = new FunFormulas();
        fun.sd();
    }
}