JAVA Apache POI Excel: add borders to cell range

In Java 1.7 or later, the standard way to do this is as follows:

import java.util.concurrent.ThreadLocalRandom;

// nextInt is normally exclusive of the top value,
// so add 1 to make it inclusive
int randomNum = ThreadLocalRandom.current().nextInt(min, max + 1);

See the relevant JavaDoc. This approach has the advantage of not needing to explicitly initialize a java.util.Random instance, which can be a source of confusion and error if used inappropriately.

However, conversely there is no way to explicitly set the seed so it can be difficult to reproduce results in situations where that is useful such as testing or saving game states or similar. In those situations, the pre-Java 1.7 technique shown below can be used.

Before Java 1.7, the standard way to do this is as follows:

import java.util.Random;

/**
 * Returns a pseudo-random number between min and max, inclusive.
 * The difference between min and max can be at most
 * <code>Integer.MAX_VALUE - 1</code>.
 *
 * @param min Minimum value
 * @param max Maximum value.  Must be greater than min.
 * @return Integer between min and max, inclusive.
 * @see java.util.Random#nextInt(int)
 */
public static int randInt(int min, int max) {

    // NOTE: This will (intentionally) not run as written so that folks
    // copy-pasting have to think about how to initialize their
    // Random instance.  Initialization of the Random instance is outside
    // the main scope of the question, but some decent options are to have
    // a field that is initialized once and then re-used as needed or to
    // use ThreadLocalRandom (if using at least Java 1.7).
    // 
    // In particular, do NOT do 'Random rand = new Random()' here or you
    // will get not very good / not very random results.
    Random rand;

    // nextInt is normally exclusive of the top value,
    // so add 1 to make it inclusive
    int randomNum = rand.nextInt((max - min) + 1) + min;

    return randomNum;
}

See the relevant JavaDoc. In practice, the java.util.Random class is often preferable to java.lang.Math.random().

In particular, there is no need to reinvent the random integer generation wheel when there is a straightforward API within the standard library to accomplish the task.

Here is a simple non-recursive solution that just concatenates the elements of each combination:

public static List<string> GetAllPossibleCombos(List<List<string>> strings)
{
    IEnumerable<string> combos = new [] { "" };

    foreach (var inner in strings)
        combos = from c in combos
                 from i in inner
                 select c + i;

    return combos.ToList();
}

static void Main(string[] args)
{
    var x = GetAllPossibleCombos(
        new List<List<string>>{
            new List<string> { "a", "b", "c" },
            new List<string> { "x", "y" },
            new List<string> { "1", "2", "3", "4" }});
}

You could generalize this to return an IEnumerable<IEnumerable<string>>, which allows the caller to apply any operation they like for transforming each combination into a string (such as the string.Join below). The combinations are enumerated using deferred execution.

public static IEnumerable<IEnumerable<string>> GetAllPossibleCombos(
    IEnumerable<IEnumerable<string>> strings)
{
    IEnumerable<IEnumerable<string>> combos = new string[][] { new string[0] };

    foreach (var inner in strings)
        combos = from c in combos
                 from i in inner
                 select c.Append(i);

    return combos;
}

public static IEnumerable<TSource> Append<TSource>(
    this IEnumerable<TSource> source, TSource item)
{
    foreach (TSource element in source)
        yield return element;

    yield return item;
}

static void Main(string[] args)
{
    var combos = GetAllPossibleCombos(
        new List<List<string>>{
            new List<string> { "a", "b", "c" },
            new List<string> { "x", "y" },
            new List<string> { "1", "2", "3", "4" }});

    var result = combos.Select(c => string.Join(",", c)).ToList();
}