How in Java randomly shuffle an array or List? This common problem can be solved with Java Collections Framework as in the following example.

In java.util.Collections class there are two methods to do shuffling:

1. Collections.shuffle(List) – uses new Random() as randomness source
2. Collections.shuffle(List, Random) – for custom Random source

Both shuffle a List as can be seen in the following Java example:

package com.farenda.java;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Random;

public class CollectionsShuffle {

    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
        System.out.println("Numbers: " + numbers);

        Collections.shuffle(numbers);
        System.out.println("Shuffled: " + numbers);

        Collections.shuffle(numbers);
        System.out.println("Shuffled again: " + numbers);

        Collections.shuffle(numbers, new Random());
        System.out.println("Shuffled with Random: " + numbers);
    }
}

To shuffle an array use Arrays.asList() to use it as an ArrayList and the use Collections.shuffle(List) as in the example. Pretty simple, no? :-)

Every invocation of the method does shuffling as can be seen in the following output:

Numbers: [1, 2, 3, 4, 5]
Shuffled: [1, 3, 4, 5, 2]
Shuffled again: [2, 1, 3, 5, 4]
Shuffled with Random: [1, 2, 4, 3, 5]