How to Shuffle Deck of Cards in Java

1. Shuffle the ArrayList Using the Traditional Loop in Java
2. Shuffle the ArrayList Using the Collections shuffle Function in Java

The word shuffle literally means to arrange the elements, objects, or cards in random or undefined order. The objects list that does not follow a defined pattern is considered shuffled.

Similarly, in Java, various methods allow a user to shuffle elements. Below is the code block to demonstrate this process.

Shuffle the ArrayList Using the Traditional Loop in Java

import java.util.ArrayList;

public class Main {
  private static final int SIZE_OF_DECK = 52;

  public static void main(String[] args) {
    ArrayList<Integer> cardsDeck = new ArrayList<>();
    for (int i = 0; i < SIZE_OF_DECK; ++i) {
      cardsDeck.add(i);
    }
    System.out.println("Deck Of Cards:" + cardsDeck);
    ArrayList<Integer> shuffledDeck = new ArrayList<>();
    while (cardsDeck.size() > 0) {
      int index = (int) (Math.random() * cardsDeck.size());
      int removedCard = cardsDeck.remove(index);
      shuffledDeck.add(removedCard);
    }
    System.out.println("Shuffled Cards" + shuffledDeck);
  }
}

In the code above, an ArrayList is instantiated. The instance created helps in adding elements into the ArrayList. The list is filled with sequential order from 0 to 51 values. The deck size is a static variable declared at the class level.

The for loop checks for the condition, wherein the initial value is checked with the static deck size variable. Now to shuffle the initial list, another empty ArrayList is created. We will apply a while loop with a condition where the deck size must be greater than zero value.

The idea to keep this condition is because the element from one deck is removed and placed in another shuffled list. Hence, the original size keeps on decreasing and finally becomes zero.

Within the loop, we use the random function to find the index first. The static method is present in the Math class and directly gets called by the class name. The utility function generates a random number ranging between 0.0 and 1.0. The method always returns a double value.

To calculate the index value, the random number formed is multiplied by the current size of the array list. Hence, generating a random index each time.

Now, the index formed is used to remove the element at the index in the original deck. The remove method is from the ArrayList class, and it removes the particular value at the index. It takes the index or the position in the list to be removed.

The function also returns the element at the index in the list. The object to be removed gets added to the new shuffled list. The loop continues, where one element now reduces the size of the original deck. Hence, when it iterates for the next time, a new index will be generated. And each time, a value is removed and appended in the shuffled list.

Below is the output of the given code block.

Deck Of Cards:[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51]
Shuffled Cards[51, 34, 28, 47, 14, 49, 42, 50, 26, 0, 44, 43, 2, 36, 30, 8, 46, 11, 21, 23, 7, 4, 33, 41, 32, 1, 20, 3, 10, 18, 6, 40, 29, 24, 31, 13, 45, 39, 22, 15, 27, 48, 9, 5, 25, 12, 38, 35, 37, 17, 16, 19]

Shuffle the ArrayList Using the Collections shuffle Function in Java

Below is the code block demonstrating the shuffling using the shuffle method.

import java.util.ArrayList;
import java.util.Collections;

public class Main {
  public static void main(String[] args) {
    int deckSize = 52;
    ArrayList<Integer> list = new ArrayList<>();
    for (int i = 0; i < deckSize; ++i) {
      list.add(i);
    }
    System.out.println("Sequential List: " + list);
    Collections.shuffle(list);
    System.out.println("Shuffled List: " + list);
  }
}

In the code block, the procedure of ArrayList instantiation is the same. The addition of elements in the list is done similarly to the one in the first code. We print the ordered list to the console using the println method.

Now, the shuffle function of Collections is called. This method performs permutation and randomly computes the values in the given list. This function dumps the randomly selected element back into the list. It throws the UnsupportedOperationException when the list provided does not support the set operation. The randomly formed list is printed in the console along with the original lists.

Sequential List[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51]
Shuffled List[46, 5, 4, 0, 19, 11, 13, 21, 1, 38, 36, 29, 31, 35, 48, 22, 49, 6, 14, 51, 3, 47, 16, 12, 42, 27, 50, 40, 26, 30, 33, 9, 43, 39, 2, 10, 28, 44, 8, 24, 41, 32, 25, 45, 34, 7, 23, 15, 18, 20, 37, 17]