Lösungsvorschlag: U = R * I

import java.util.Scanner;
 
/**
 * Calculate  U = R * I
 *
 * @author Kevin Maurizi
 */
public class URI {
 
    static Scanner scanner;
    /**
     * constructor
     */
    public URI() {
        scanner = new Scanner(System.in);
    }
 
    /**
     * starts the execution
     *
     * @param args
     *            command line arguments
     */
    public static void main(String[] args) {
        URI program = new URI();
 
        program.calculate();
        scanner.close();
    }
 
    /**
     * run the userdialog and calculate
     */
    private void calculate() {
 
        String spannung;
        String widerstand;
        String stromstaerke;
        String resultat = "";
 
        System.out.print("U > ");
        spannung = scanner.nextLine();
 
        System.out.print("R > ");
        widerstand = scanner.nextLine();
 
        System.out.print("I > ");
        stromstaerke = scanner.nextLine();
 
        // Verarbeitung
        if(spannung.equals("?")){
            double i = Double.valueOf(stromstaerke);
            double r = Double.valueOf(widerstand);
            resultat = "Resultat U = " + i * r + " V";
        }
        if(stromstaerke.equals("?")){
            double u = Double.valueOf(spannung);
            double r = Double.valueOf(widerstand);
            resultat = "Resultat I = " + u / r + " A";
        }
        if(widerstand.equals("?")){
            double u = Double.valueOf(spannung);
            double i = Double.valueOf(stromstaerke);
            resultat = "Resultat R = " + u / i + " Ohm";
        }
 
        // Ausgabe
        System.out.println(resultat);
    }
}