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

java, random-access, shuffle

Solution

`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 (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] + " "); 
} 

Problem

Original source