为什么Collections.shuffle（）为我的数组失败？

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 Integer s. 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 (ie 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] + " "); 
} 

Answer 2

Try adding this line of code to your test:

List l=Arrays.asList(arr);
System.out.println(l);

You will see you are printing out a single element List .

Using Arrays.asList on a primitive array cause asList to treat the int[] as a single object rather than an array. It returns a List<int[]> instead of a List<Integer> . So, you are basically shuffling a single element List and so nothing really gets shuffled.

Notice that some of the answers already given are wrong because asList returns a List backed by the original array, nothing gets copied - all changes are reflected in the orginal array.

Answer 3

That doesn't work because the call to shuffle is operating on the List returned by Arrays.asList , not on the underlying array. Thus, when you iterate over the array to print out the values, nothing has changed. What you want to do is save a reference to the List returned by Arrays.asList , and then print out the values of that List (rather than the values of the array) after you shuffle it.

Answer 4

Store the list resturned by Arrays.asList and shuffle that...

List myShuffledList = Arrays.asList(arr);
Collections.shuffle(myShuffledList);