Java 10: Will Java 7's Diamond Inference Work with Local Type Inference?

Question

From JEP 286, we see that we'll be able to utilize local type inference (var) in JDK 10 (18.3). The JEP states that the following compiles, which is expected:

var list = new ArrayList<String>();  // infers ArrayList<String>

I'm curious to know what would happen if we attempt the following:

var list = new ArrayList<>();

Will what I proposed in the second snippet even compile? If so (which I doubt), would the ArrayList accept Object as its generic type?

I'd try this myself, but I don't have access to any machines which I can install early releases on.

Thanks!

question from:https://stackoverflow.com/questions/48428434/java-10-will-java-7s-diamond-inference-work-with-local-type-inference

Answer 1

Yes, var and the diamond operator can be combined together. The compiler will infer the most specific generic type:

var list = new ArrayList<>(); // Infers ArrayList<Object>
var list = new ArrayList<>(List.of(1, 2, 3)); // Infers ArrayList<Integer>

And you can even combine them with an anonymous class:

var list = new ArrayList<>() {};