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

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.

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] + " "); 
}