Esben86 Esben86 - 1 year ago 51
Java Question

Manual list shuffling in Java produces same result each time

I am trying to create a method for shuffling a list manually. The list is shuffled, but the same result is produced every time. Code example below:

package ch11;
import java.util.ArrayList;
import java.util.Arrays;
public class Chapter_11_E07_ShuffleArrayList {

public static void main(String[] args) {

Integer[] array = {1, 2, 3, 4, 5, 6, 7, 8};

ArrayList<Integer> intList = new ArrayList<>(Arrays.asList(array));

System.out.println("Before shuffle: ");
for (Integer x: intList)
System.out.print(x + " ");


System.out.println("\nAfter shuffle: ");
for (Integer x: intList)
System.out.print(x + " ");


public static void shuffle(ArrayList<Integer> intList) {
// Simple solution
// java.util.Collections.shuffle(intList);

// Manual shuffle
for (Integer x: intList) {

int newIndex = (int) Math.random() * intList.size();

Integer temp = intList.get(x);
intList.set(x, intList.get(newIndex));
intList.set(newIndex, temp);


It seems to work to some extent, but is
Math.random * intList.size()
producing the same random index every time? Inputs are highly appreciated!

Answer Source

This is because

int newIndex = (int) Math.random() * intList.size();

is not parenthesized properly. It should be

int newIndex = (int)(Math.random() * intList.size());

To avoid simple errors like this, make new Random object, and call nextInt(intList.size()).