Get random strings from a list

Question

I have an ArrayList which is defined outside the main method, just inside the class StringRandomize:

 public static ArrayList<String> countries = new ArrayList<String>();

I also initialized a random object.

 Random obj = new Random();

Then I add some Strings to the list:

    StringRandomize.countries.add("USA");
    StringRandomize.countries.add("GB");
    StringRandomize.countries.add("Germany");
    StringRandomize.countries.add("Austria");
    StringRandomize.countries.add("Romania");
    StringRandomize.countries.add("Moldova");
    StringRandomize.countries.add("Ukraine");

How do I make those strings appear randomly? I need output like "Germany", "Moldova" and so on.
I need exactly the strings in the output, not their IDs. Thanks for your help.

If you want to print the all content (and not just fetch a random element), you can also consider shuffling the list with Collections.shuffle. — Alexis C.
– Alexis C., Commented Feb 12, 2016 at 22:16

fanton · Accepted Answer · 2016-02-12 22:12:19Z

3

You probably want to use something like:

countries.get(Math.abs(new Random().nextInt()) % countries.size());

Or, to avoid creating a new Random object every time, you could use the same one:

Random gen = new Random();
for (int i = 1; i < 10; i++) {
    System.out.println(countries.get(Math.abs(gen.nextInt()) % countries.size()));
}

answered Feb 12, 2016 at 22:12

fanton

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3 Comments

Alexis C. Over a year ago

or .nextInt(countries.size()).

AmanKumar Over a year ago

can we do like this countries.get(obj.nextInt()); ?

fanton Over a year ago

@AlexisC: That's true. You can do that, also. @AmanKumar: You can, but pay attention to avoid a potential indexOutOfBounds exception. You have to make sure that obj.nextInt() is positive and less than countries.size().

heniv181 · Accepted Answer · 2016-02-12 22:23:41Z

1

I would use Collections.shuffle(countries) if you wanted a randomized List.

Else a new Random().nextInt(max) like Flavius described.

answered Feb 12, 2016 at 22:23

heniv181

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Comments

Simply Me · Accepted Answer · 2016-02-13 08:00:57Z

1

static void shuffleArray(string[] ar)
  {
    //set the seed for the random variable
    Random rnd = ThreadLocalRandom.current();
    //go from the last element to the first one.
    for (int i = ar.size()- 1; i > 0; i--)
    {
      //get a random number till the current position and simply swap elements
      int index = rnd.nextInt(i + 1);
      // Simple swap
      int a = ar[index];
      ar[index] = ar[i];
      ar[i] = a;
    }
  }

This way you shuffle the entire array and get the values in a random order but NO duplicate at all. Every single element changes position, so that no matter what element (position) you pick, you get a country from a random position. You can return the entire vector, the positions are random.

edited Feb 13, 2016 at 8:00

answered Feb 12, 2016 at 22:13

Simply Me

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1 Comment

Nicolai Stoianov Over a year ago

Could you comment it please? And by the way, I get the " Cannot resolve method 'length()' " error.

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