Junit - testing a different class

Hi I'm trying to get my head around Junit testing and I can't find a way to test another class without copy pasting parts of it in. say I want to test this:

import java.io.*;

public class Calculator {

public static void main(String[] args) {
    String s1 = getInput("Enter a numeric value: ");
    String s2 = getInput("Enter a numeric value: ");
    String op = getInput("Enter 1=ADD, 2=Subtract, 3=Multiply, 4=Divide ");

    int opInt = Integer.parseInt(op);
    double result = 0;

    switch (opInt) {
    case 1:
        result = addValues(s1, s2);
        break;
    case 2:
        result = subtractValues(s1, s2);
        break;
    case 3:
        result = multiplyValues(s1, s2);
        break;
    case 4:
        result = divideValues(s1, s2);
        break;

    default:
        System.out.println("You entered an incorrect value");
        return;
    }

    System.out.println("The answer is " + result);

}

private static double divideValues(String s1, String s2) {
    double d1 = Double.parseDouble(s1);
    double d2 = Double.parseDouble(s2);
    double result = d1 / d2;
    return result;
}

private static double multiplyValues(String s1, String s2) {
    double d1 = Double.parseDouble(s1);
    double d2 = Double.parseDouble(s2);
    double result = d1 * d2;
    return result;
}

private static double subtractValues(String s1, String s2) {
    double d1 = Double.parseDouble(s1);
    double d2 = Double.parseDouble(s2);
    double result = d1 - d2;
    return result;
}

private static double addValues(String s1, String s2) {
    double d1 = Double.parseDouble(s1);
    double d2 = Double.parseDouble(s2);
    double result = d1 + d2;
    return result;
}

private static String getInput(String prompt) {
    BufferedReader stdin = new BufferedReader(
            new InputStreamReader(System.in));

    System.out.print(prompt);
    System.out.flush();

    try {
        return stdin.readLine();
    } catch (Exception e) {
        return "error: " + e.getMessage();
    }
}

Is there any way I can set up the JUnit case test to check parts of this without copy and pasting it in for every test or modifying the orginal class. am I missing something or is this something Junit can't do?