Java Collections shuffle

Table of Contents

Syntax
Internal working of Java collections shuffle

Shuffle method is to permute passed list with default or specified randomness.

Syntax

public static void shuffle(List<?> list)

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

Example :
Let’s see both the function with the help of example.

package org.arpit.java2blog;

import java.util.Arrays;

import java.util.Collections;

import java.util.List;

import java.util.Random;

public class ShuffleMain {

public static void main(String[] args)

{

List<String> listOfEmployeeNames = Arrays.asList("John","Martin","Mohan","Mary","Tom");

System.out.println("==========================");

System.out.println("Before shuffling");

System.out.println(listOfEmployeeNames);

Collections.shuffle(listOfEmployeeNames);

System.out.println("==========================");

System.out.println("After shuffling");

System.out.println(listOfEmployeeNames);

listOfEmployeeNames = Arrays.asList("John","Martin","Mohan","Mary","Tom");

Collections.shuffle(listOfEmployeeNames, new Random(3));

System.out.println("==========================");

System.out.println("After shuffling with randomness of 3");

System.out.println(listOfEmployeeNames);

}

When you run above program, you will get below output:

==========================
Before shuffling
[John, Martin, Mohan, Mary, Tom] ==========================
After shuffling
[Mohan, Martin, Mary, Tom, John] ==========================
After shuffling with randomness of 3
[Mary, Martin, John, Mohan, Tom]

Internal working of Java collections shuffle

Java Collections shuffle interally does Fisher-Yates shuffle.Here is the implementation of shuffle method provided by JDK.

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

int size = list.size();

if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) { for (int i=size; i>1; i--)

swap(list, i-1, rnd.nextInt(i));

} else {

Object arr[] = list.toArray();

// Shuffle array

for (int i=size; i>1; i--)

swap(arr, i-1, rnd.nextInt(i));

// Dump array back into list

ListIterator it = list.listIterator();

for (int i=0; i<arr.length; i++) {

it.next();

it.set(arr[i]);

}

Here is implementation of swap method.

private static void swap(Object[] x, int a, int b) {

Object t = x[a];

x[a] = x[b];

x[b] = t;

}

That’s all about Java collections shuffle method.

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