Program based on a physics concept
First, the findX functions don't need to initialise the variable oneUpon inside. It's also unclear, anyway.


public static double findU(double v, double f) {

    double oneUpon;

    oneUpon = 1/f - 1/v;
    return 1/oneUpon;

}

public static double findV(double u, double f){

    double oneUpon;

    oneUpon = 1/f - 1/u;
    return 1/oneUpon;
}

public static double findF(double u, double v){

    double oneUpon = 1/u + 1/v;
    return 1/oneUpon;
}



Just return the variable directly.

I'd also like to point out that findU and findV are identical in function, just named differently.

Consider the following instead:

public static double findUV(double uv, double f) {
    return 1/(1/f - 1/uv);
}

public static double findF(double u, double v){
    return 1/(1/u + 1/v);
}





    System.out.println("Hello! This is a calculator based on the Mirror Formula. \n");
    System.out.println("The mirror formula is a relation between the object distance, the image distance \n and the focal length.");
    System.out.println("The formula is : 1/u + 1/v = 1/f.  (u=distance of object, v=distance of image, f=focal length.)");
    System.out.println("It is applicable for both convex mirrors and concave mirrors.");
    System.out.println();
    System.out.println("This calculator can calculate :");
    System.out.println();
    System.out.println("1. Distance of the object from the pole of the spherical mirror.");
    System.out.println("2. Distance of the image from the pole of the spherical mirror.");
    System.out.println("3. Focal length");
    System.out.println("[If you have 2 of the values.]");



This is not how you print large blocks. Separate your strings with line separators like /n. Keep the text as a large multi-line string block, and print that out instead. It will take less maintenance and less System.out.println calls.



Feel free to take the user input as a string/letter instead of an integer, makes everything much clearer. On a related note, an exit function would be nice too.


    switch (userChoice) {

    case 1 :