Java Error: Generic array creation

Java does not allow you to create arrays of generic classes:

Workaround 1: Raw types

Set<String>[] sets = new Set[5];

Workaround 2: Use a List instead

List<Set<String>> sets = new ArrayList<>();

Workaround 3: Non-generic subclass

You can create a non-generic subclass (local if you like) that extends the generic class:

class StringHashSet extends HashSet<String> {}
Set<String>[] sets = new StringHashSet[5];

What’s the reason for this limitation?

Let’s illustrate what could happen with an example:

// Suppose this was allowed
Set<String>[] arr = new Set<String>[5];

// Arrays are covariant:
Object[] oa = arr;

// Putting a Set<Integer> in arr!
oa[0] = Collections.singleton(3);

// Looks type safe but isn't!
String s = arr[0].iterator().next();

Arrays actually have the same problem. This program does in fact even compile:

String[] arr = new String[5];
Object[] oa = arr;
oa[0] = 5; // Eeek! An Integer in a String[]?
String s = arr[0];

Here however, oa[0] = 5 throws an ArrayStoreException in runtime. In other words, when generics were introduced the designers traded some expressiveness for some added compile time type safety.