Creating a Map through an anonymous class with the <>

Question

In JDK 1.7 I can create a Collection lets for e.g. say a HashMap like this:

private HashMap<String, String> map = new HashMap<>();

With the diamond <> at the end.

But if I am creating a map like this:

private static final HashMap<String, String> MAP = new HashMap<>() {{
    put("something", "something");
}};

On the diamond compiler says that:

Cannot use ''<>'' with anonymous inner classes

I have to use: ... new HashMap<String, String>() {{.... in order the code to compile.

Why is it so? Why I can create a map and use diamond if I am creating just a new instance but the code doesn't compile if I am creating a map through an anonymous class?

I don't think it is duplicate - this question is asking WHY. The answers to the other question don't really say why Sun made that decision, just that you can't do it. — David Lavender, Feb 14 '13 at 16:05
@MrSpoon true, but Type Inference was introduced in java 7 after oracle bought sun .. :) — PermGenError, Feb 14 '13 at 16:13

score 4 · Accepted Answer · answered Feb 14 '13 at 15:56

4

This is just a guess but when you do

new HashMap<>() {{
    put("something", "something");
}};

It actually creates a subclass of HashMap I don't like this because you dirty up your classes with a bunch of subclasses. I am guessing in this case Java cannot infer what the generic diamond type is because it is being subclassed.

answered Feb 14 '13 at 15:56

Amir Raminfar

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but I think the question OP is asking is *why* is Java unable to infer it in this case? – matt b Feb 14 '13 at 15:59
@mattb *It actually creates a subclass of `HashMap`* – Luiggi Mendoza Feb 14 '13 at 16:00
Yep, but as said on http://stackoverflow.com/questions/9773733/double-brace-initialisation-anonymous-inner-class-with-diamond-operator it is also because of subclassing. That question doesn't have an answer either to why though. – Amir Raminfar Feb 14 '13 at 16:00
@AmirRaminfar Actually the accepted answer for that question does offer a reason to why: "the subclass would then be compiled as if you wrote `... extends HashMap – matts Feb 14 '13 at 16:02
@matts that's right I missed that. – Amir Raminfar Feb 14 '13 at 16:04

Creating a Map through an anonymous class with the <>

1 Answers1