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Arrays of parameterized type

by Ram Satish

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In this article, I will show you why you can’t create an array of parameterized types.

In the example below, I try creating an array of parameterized type but the compiler will treat this as an error.

Node<Integer>[] intNodeArray = new Node<Integer>[3];

Cannot create a generic array of Node<Integer>

Before we figure out why Java Generics doesn’t support array creation of parameterized types, we need to know what is it being covariant.

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Arrays are covariant

An array of supertype references is a supertype of an array of subtype references.
That is, Object[] holding references of Objects is a supertype of String[] array holding an array of string references so below is allowed by the compiler. Even runtime allows it as Object is supertype of String.

Object[] objArray = new String[3];
objArray[0] = new String();

Why creation of an array of parameterized type is not allowed?

Assuming compiler allows creation of array of parameterized type, it would be legal to assign the Node<Integer>[] array to an Object[] array since arrays are covariant.
Node<Integer>[] intNodeArray = new Node<Integer>[]{new Node(1), new Node(2)}; throws error but for now let us assume it is allowed.
Now we will assign a Node object of different type to our object array. You can see in the example below, I have assigned a Node<String> object to the first element of an object array which is supposed to be holding Node<Integer> objects. Once this is done, we will end up in an unacceptable situation.

    public void testTypedArray() {
Node<Integer>[] intNodeArray = new Node<Integer>[]{new Node(1), new Node(2)};//throws error
arrayCovariant(nodeArray);
Integer i = (Integer) nodeArray[0].getValue();
}

private void arrayCovariant(Node<Integer>[] inNodeStrArray) {
Object[] oa = inNodeStrArray;
oa[0] = new Node<String>("Will fail at runtime");
}

Integer i = (Integer) nodeArray[0].getValue() will throw ClassCastException.
In order to prevent this kind of type-unsafe situation, arrays of parameterized type are not allowed.

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