Random shuffling of an array

I need to randomly shuffle the following Array:

int[] solutionArray = {1, 2, 3, 4, 5, 6, 6, 5, 4, 3, 2, 1};

Is there any function to do that?

Solution 1:

Using Collections to shuffle an array of primitive types is a bit of an overkill...

It is simple enough to implement the function yourself, using for example the Fisher–Yates shuffle:

import java.util.*;
import java.util.concurrent.ThreadLocalRandom;

class Test
{
  public static void main(String args[])
  {
    int[] solutionArray = { 1, 2, 3, 4, 5, 6, 16, 15, 14, 13, 12, 11 };

    shuffleArray(solutionArray);
    for (int i = 0; i < solutionArray.length; i++)
    {
      System.out.print(solutionArray[i] + " ");
    }
    System.out.println();
  }

  // Implementing Fisher–Yates shuffle
  static void shuffleArray(int[] ar)
  {
    // If running on Java 6 or older, use `new Random()` on RHS here
    Random rnd = ThreadLocalRandom.current();
    for (int i = ar.length - 1; i > 0; i--)
    {
      int index = rnd.nextInt(i + 1);
      // Simple swap
      int a = ar[index];
      ar[index] = ar[i];
      ar[i] = a;
    }
  }
}

Solution 2:

Here is a simple way using an ArrayList:

List<Integer> solution = new ArrayList<>();
for (int i = 1; i <= 6; i++) {
    solution.add(i);
}
Collections.shuffle(solution);

Solution 3:

Here is a working and efficient Fisher–Yates shuffle array function:

private static void shuffleArray(int[] array)
{
    int index;
    Random random = new Random();
    for (int i = array.length - 1; i > 0; i--)
    {
        index = random.nextInt(i + 1);
        if (index != i)
        {
            array[index] ^= array[i];
            array[i] ^= array[index];
            array[index] ^= array[i];
        }
    }
}

or

private static void shuffleArray(int[] array)
{
    int index, temp;
    Random random = new Random();
    for (int i = array.length - 1; i > 0; i--)
    {
        index = random.nextInt(i + 1);
        temp = array[index];
        array[index] = array[i];
        array[i] = temp;
    }
}

Solution 4:

Collections class has an efficient method for shuffling, that can be copied, so as not to depend on it:

/**
 * Usage:
 *    int[] array = {1, 2, 3};
 *    Util.shuffle(array);
 */
public class Util {

    private static Random random;

    /**
     * Code from method java.util.Collections.shuffle();
     */
    public static void shuffle(int[] array) {
        if (random == null) random = new Random();
        int count = array.length;
        for (int i = count; i > 1; i--) {
            swap(array, i - 1, random.nextInt(i));
        }
    }

    private static void swap(int[] array, int i, int j) {
        int temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    }
}