解决我的初始越界异常后，如何将a与b或c相乘

Question

I am new to Java programming and I want to make a program of calculating the speed of a car in relation of the gear and the RPM.

I searched different sites or topics, but couldn't find something. Maybe the answer was there, but I haven't seen it :D.

A little but not complete program for example:

import java.util.Scanner;

public class Speed {
    public static void main(String[] args) {

        double Gear, MPH, RPM
        double TireDia = 78;
        double Pi = 3.1415;
        double RatioGearOne = 3.4;
        double RatioGearTwo = 2.5;
        double RatioGearThree = 1.6;

        Scanner input = new
        Scanner (System.in);

        System.out.println("What gear is it in?");
        Gear = input.nextDouble();

        System.out.println("How many RPM are shown?");
        RPM = input.nextDouble();

        MPH = Gear * RPM * Pi * TireDia * 60 / 1000000;
        System.out.println("The car is going: " + MPH + " MPH");
    }
}    

Like I asked, I want that you can enter Gear 1 or Gear 2 or Gear 3 multiplied by PRM, Pi, etc to display me the MPH. I read that you can do this with an array too, but i want it line that for now, cause I understand it a bit more at the time.

I hope you can help me.

UPDATE:

I got it working as I want it for now and cleaned it a bit with your suggestions. Now I need to figure out how I get around the out of bounds error, but havent figured it out how, (searching for hours by now) and get the speed in a whole number. Thanks for help so far.

This is my current Program:

import java.util.Scanner;

public class geschwindigkeitAuto {
public static void main(String[] args) {

    double gang, kmh, upm;

    Scanner input = new Scanner (System.in);

    double[] übersetzungen = {3.7, 1.95, 1.3, 1.03, 0.84, 0.68};

    System.out.println("Welcher Gang ist Eingelegt? (wähle 1 bis 6): ");
    gang = input.nextInt();

    System.out.println("Wieviele Umdrehungen werden angezeigt? (wähle 0-6500): ");
    upm = input.nextInt();

    kmh = upm / übersetzungen[(int)gang -1] / 3.62 * 68 * 3.141 * 60 / 100000;
    System.out.print("Die Geschwindigkeit des Autos beträgt: " + kmh + " kmh");
}
}

UPDATE 2

I have searched more and watched some videos for the OutOfBoundExcenption to avoid and I have found a solution so that I cannot input less than 1 or more than 6 to my program without failing. The only thing is now, how to tell the user to input a different number after he made a wrong choise instead of fist going through the whoule sequenz.

import java.util.Scanner;

public class geschwindigkeitAuto {
public static void main(String[] args) {
    double gang, kmh = 0, upm;

    Scanner input = new Scanner (System.in);

    double[] übersetzungen = {3.7, 1.95, 1.3, 1.03, 0.84, 0.68};

    System.out.println("Welcher Gang ist Eingelegt? (wähle Gang 1 bis 6): ");
    gang = input.nextInt();

    System.out.println("Wieviele Umdrehungen werden angezeigt? (wähle 0-6500): ");
    upm = input.nextInt();

    try {
        kmh = upm / übersetzungen[(int)gang -1] / 3.62 * 68 * 3.141 * 60 / 100000; } 
    catch(ArrayIndexOutOfBoundsException ex) {
        System.out.println("Du hast einen nicht vorhandenen Gang gewählt! " 
        + "Es gibt nur 6 Gänge zur Auswahl" + "\n"); }

    System.out.println("Die Geschwindigkeit des Autos beträgt: " + kmh + " kmh" + "\n");

    System.out.print("Noch eine Berechnung? (J/N) : ");
    System.out.printf("\n");
    String var= input.next();
        if(var.equalsIgnoreCase("J")) {
        main(null);
    }
}
}

Answer 1

If you're adamant that you don't want to use an array then you can use a switch statement:

switch(gear) {
     case 1: ratioGearOne * ...
     case 2: ratioGearTwo * ...
     ...
}

etc etc. You'll also need a default: that's run whenever the input doesn't conform to one of the cases (eg print an error).

If you were to use an array you could do it like this:

double[] ratios = { 3.4, 2.5, 1.6 }

Then you could calculate the MPH like so:

MPH = ratios[gear - 1] * ...

Note you need to subtract one from the gear since array indexes start from 0. I believe you'd also need to change your gear variable to an int (use nextInt()) since you can't have half an index.

As other commenters have said I'd strongly recommend reading up on Java conventions (particularly variable naming) before getting into any bad habits.