Why does Collections.shuffle() fail for my array?

Question

Why does my code not work?

package generatingInitialPopulation;

import java.util.Arrays;
import java.util.Collections;

public class TestShuffle {
    public static void main(String[] args) {
        int[] arr = new int[10];

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

        Collections.shuffle(Arrays.asList(arr));

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

The result is: 0 1 2 3 4 5 6 7 8 9.

I was expecting a randomly shuffled sequence.

Answer 1

Arrays.asList() can't be applied to arrays of primitive type as you expect. When applied to int[], Arrays.asList() produces a list of int[]s instead of list of Integers. Therefore you shuffle a newly created list of int[].

This is a subtle behaviour of variadic arguments and generics in Java. Arrays.asList() is declared as

public static <T> List<T> asList(T... a)

So, it can take several arguments of some type T and produce a list containing these arguments, or it can take one argument of type T[] and return a list backed by this array (that's how variadic arguments work).

However, the latter option works only when T is a reference type (i.e. not a primitive type such as int), because only reference types may be used as type parameters in generics (and T is a type parameter).

So, if you pass int[], you get T = int[], and you code doesn't work as expected. But if you pass array of reference type (for example, Integer[]), you get T = Integer and everything works:

Integer[] arr = new Integer[10]; 

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

Collections.shuffle(Arrays.asList(arr)); 

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