Java unbounded wildcard misunderstanding

Asked Jul 05 '18 at 18:13

Active Jul 05 '18 at 18:17

Viewed 43 times

Let’s say I define a collection of parametrized types like this:

Deque<Node<Integer>> nodes = new LinkedList<>();

nodes.add(new Node(5));

Why is it an error to pass that reference to a method with an unbounded wildcard in the signature like this:

static void printNodes(Collection<Node<?>> nodes) {...}

edited Jul 05 '18 at 18:17

asked Jul 05 '18 at 18:13

Node1895

You should also mention the error. – NiVeR Jul 05 '18 at 18:16
1

Because generics are invariant, see: https://stackoverflow.com/q/2745265 You'd need to change the parameter type to `Collection extends Node>>` – Jorn Vernee Jul 05 '18 at 18:17
1

You could add a `Node` to a `Collection>`, so a `Collection>` is not a `Collection>`. – Andy Turner Jul 05 '18 at 18:21

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