How to pick random element from 2d String array in java

String[][] cardDeck = {
        { "AceH", "2H", "3H", "4H", "5H", "6H", "7H", "8H", "9H", "10H", "JackH", "QueenH", "KingH" },
        { "AceD", "2D", "3D", "4D", "5D", "6D", "7D", "8D", "9D", "10D", "JackD", "QueenD", "KingD" },
        { "AceS", "2S", "3S", "4S", "5S", "6S", "7S", "8S", "9S", "10S", "JackS", "QueenS", "KingS" },
        { "AceC", "2C", "3C", "4C", "5C", "6C", "7C", "8C", "9C", "10C", "JackC", "QueenC", "KingC" }};
    Random random = new Random();
    int dim1Pos = random.nextInt(cardDeck.length); // pick random position for 1st dimension (row)
    int dim2Pos = random.nextInt(cardDeck[dim1Pos].length); // pick random position for 2nd dimension (column) for previously selected row
    String randomString = cardDeck[dim1Pos][dim2Pos];
    System.out.println(randomString);

How about this:

String[][] cardDeck = { 
    { "AceH", "2H", "3H", "4H", "5H", "6H", "7H", "8H", "9H", "10H", "JackH", "QueenH", "KingH" }, 
    { "AceD", "2D", "3D", "4D", "5D", "6D", "7D", "8D", "9D", "10D", "JackD", "QueenD", "KingD" },
    { "AceS", "2S", "3S", "4S", "5S", "6S", "7S", "8S", "9S", "10S", "JackS", "QueenS", "KingS" },
    { "AceC", "2C", "3C", "4C", "5C", "6C", "7C", "8C", "9C", "10C", "JackC", "QueenC", "KingC" }};
int random_y = (int)(Math.random() * cardDeck.length);
int random_x = (int)(Math.random() * cardDeck[random_y].length);
System.out.println(cardDeck[random_y][random_x]);

Math.random() returns a random DECIMAL between 0 and 1, so if we multiply it by some number a, we get a random decimal between 0 and a (non-inclusive). Converting it to int, we then get a random integer between 0 and a. And that's exactly what we need to get a random index!

But in this case, we'll need 2 random integers, since this is a 2D array. You can call them random_x and random_y, since they'll correspond to the random column and random row respectively. So we can access the random element using cardDeck[random_y][random_x].

If you're still confused as to why we multiply it by the length of the array, consider this: to get a random row, we need to access cardDeck[i], where i is an integer between 0 and cardDeck.length. In this particular row of the matrix, we want a random element. So we access it using cardDeck[i][j], where j is an integer between 0 and cardDeck[i].length.

Instead of using the Math.random() method, you could just use the Random class.

String[][] cardDeck = { 
    { "AceH", "2H", "3H", "4H", "5H", "6H", "7H", "8H", "9H", "10H", "JackH", "QueenH", "KingH" }, 
    { "AceD", "2D", "3D", "4D", "5D", "6D", "7D", "8D", "9D", "10D", "JackD", "QueenD", "KingD" },
    { "AceS", "2S", "3S", "4S", "5S", "6S", "7S", "8S", "9S", "10S", "JackS", "QueenS", "KingS" },
    { "AceC", "2C", "3C", "4C", "5C", "6C", "7C", "8C", "9C", "10C", "JackC", "QueenC", "KingC" }};
Random random = new Random();
int randomX = random.nextInt(cardDeck.length);
int randomY = random.nextInt(cardDeck[0].length);
System.out.println(cardDeck[randomX][randomY]);

Random is a class which can generate random numbers, in fact, the Math.random() method uses the Random class itself. random.nextInt(cardDeck.length); generates a random integer between 0(inclusive) and the number of rows(exclusive), which is a valid index. Similiarly, random.nextInt(cardDeck[0].length); will return a random integer between 0(inclusive) and the number of columns(exclusive), which too is the valid index.

You could use Streams to collect all elements to a list, and then shuffle the list:

List<String> list = Arrays.stream(cardDeck).      //stream to String[]
                    flatMap(x->Arrays.stream(x)). //stream to String
                    collect(Collectors.toList()); //collect to a List 

Collections.shuffle(list);    //shuffle collection
String randomElement = list.get(0); //get an arbitrary element 

You could also use Streams.skip(n) :

Optional<String> randomElement =  Arrays.stream(cardDeck).  //stream to String[]
                                 flatMap(x->Arrays.stream(x)). //stream to String
                                 skip(new Random().nextInt(cardDeck.length*cardDeck[0].length ))//remove random number of elements 
                                .findFirst();