Question

I’m just starting with Java, so I’m transcribing some of my Ruby stuff. I have a Ruby function like this, which generates something that looks like an IPv6 address:

def generate_ip()
    list = ["a", "b", "c", "d", "e", "f", "1", "2", "3", "4", "5", "6", "7", "8", "9", "0"]
    address = ""
    8.times do
        4.times do
            address << list.shuffle[5]
        end
        address << ":"
    end
    return address[0..-2].to_sym
end

This method simply takes an array of strings and then repeatedly jumbles them up, choosing the 5th element each time to construct the returned string.

As far as I can tell, there’s no “shuffle()” method for String Arrays in Java, so I figured I’d write my own. But then I started to get completely mired in sub-problems, such as:

1. Apparently there’s no built-in way to remove items from a String Array, which seems strange — is that correct? I saw several other SO posts where the responses for this features were a 10-line block of code. So I decided to use ArrayLists, since according to the doc, they have sane methods like size(), get(), and remove(), although…

2. …I haven’t found a way to instantiate them using a literal, or to add multiple items with a single method call. Every tutorial I see (including the official Oracle Java Tutorial) uses multiple calls to ArrayList.add(). That can’t be the way to do it, can it?

3. etc.

So before I go spiraling down into sub-problem land and asking 50 different questions, I thought I’d ask how to solve the most important problem. Is there a simple way to translate the above Ruby code into Java?

FWIW, my in-progress “shuffle” method is below. I’m just looping array.length times, copying/removing a random element from the old array to a new array each time.

public ArrayList<String> shuffle(ArrayList<String> array) {
        Random generator = new Random();
        ArrayList<String> shuffled = new ArrayList<String>();

        for (int i = 0; i > array.size(); i++) {
            int index = generator.nextInt(array.size());

            String popped = array.get(index);
            array.remove(index);

            shuffled.add(popped);
        }


        return shuffled;
    }

Answer 1

If you create a List (either ArrayList or LinkedList), you can use Collections.shuffle()

ArrayList and LinkedList are both implementations of the List interface. I suspect either would be ok for your needs here, although the ArrayList would be preferable (being backed by an array). Here’s the Java List tutorial.

Arrays are of a fixed length in Java, whereas a List is variable in size, and so the List is more appropriate in general for your above conversion. To create a list with a given set of starting elements, see Arrays.asList() e.g.

List stooges = Arrays.asList("Larry", "Moe", "Curly");