Collections.shuffle() Method in Java with Examples

The shuffle() method of the Collections class is used to randomly rearrange the elements of a list. It belongs to the java.util package and changes the order of elements in the original list.

• Collections.shuffle() randomly rearranges the elements of a list.
• It can use either the default source of randomness or a user-provided Random object.

Working Of Collections.shuffle()

The shuffle() method randomly permutes the elements of the list.

• It processes the list from the last element toward the beginning.
• At each position, it selects a random element from the remaining portion of the list.
• The selected element is then swapped with the current position.

Syntax

There are two overloaded versions of the shuffle() method.

1. Using Default Randomness:

public static void shuffle(List<?> list)

2. Using User-Provided Randomness:

public static void shuffle(List<?> list, Random rnd)

Example: Shuffle a List using Collections.shuffle()

import java.util.*;

class ShuffleDemo {

    public static void main(String[] args)
    {

        // Create a list
        ArrayList<String> fruits = new ArrayList<>();

        fruits.add("Apple");
        fruits.add("Mango");
        fruits.add("Orange");
        fruits.add("Banana");
        fruits.add("Grapes");

        // Print the original list
        System.out.println("Original List: " + fruits);

        // Shuffle the list
        Collections.shuffle(fruits);

        // Print the shuffled list
        System.out.println("Shuffled List: " + fruits);
    }
}

Original List: [Apple, Mango, Orange, Banana, Grapes]
Shuffled List: [Orange, Apple, Mango, Banana, Grapes]

Explanation: The list initially contains five fruits in a fixed order. The Collections.shuffle(fruits) method randomly changes their positions. Since the method uses randomization, the shuffled order may be different each time the program runs.

Example: Shuffle a List Using Random

import java.util.*;

class ShuffleRandomDemo {

    public static void main(String[] args)
    {

        ArrayList<Integer> numbers = new ArrayList<>();

        numbers.add(10);
        numbers.add(20);
        numbers.add(30);
        numbers.add(40);
        numbers.add(50);

        System.out.println("Original List: " + numbers);

        // Create a Random object with a fixed seed
        Random random = new Random(10);

        // Shuffle the list using the Random object
        Collections.shuffle(numbers, random);

        System.out.println("Shuffled List: " + numbers);
    }
}

Original List: [10, 20, 30, 40, 50]
Shuffled List: [50, 30, 10, 20, 40]

Explanation: The Random object is created with the seed 10 and passed to Collections.shuffle(). Using the same seed produces the same shuffle sequence, making the result useful when a repeatable result is required.