java - Local Type Inference vs Instance

Question

I've tried to scan JEP-286 about local type inference. I see that this works only for local variables - understood. So this does work indeed:

public class TestClass {
    public static void main(String [] args){
        var list = new ArrayList<>();
        list.add("1");
        System.out.println(list.get(0)); // 1
    }  
}

I do see that this on the other hand does not compile:

public class TestClass {
    public var list = new ArrayList<>();
    public static void main(String [] args){

    }
}

It's obvious that it does not, since the JEP says so. Now my question:

It makes perfect sense for a public/protected member declared as var to fail, at least IMO. But why does it not compile even if it's private? I can only assume that you can still get a hold of that variable via reflection (and I can't get local fields like this)... And getting that variable would require a cast, well, a very confused cast probably.

Answer 1

The motivation for forbidding type inference for fields and method returns is that APIs should be stable; field access and method invocation are linked by descriptor at runtime, so things that cause subtle changes to inferred types could cause existing compiled clients to break in terrible ways if a change to the implementation caused the inferred type to change (modulo erasure.) So using this for implementation, but not for API, is a sensible guiding principle.

It is reasonable to ask "so, what about private fields and methods?" And indeed, we could well have chosen to do that. Like all design decisions, this is a tradeoff; it would enable inference to be used in more places, in exchange for more complexity in the user model. (I don't care as much about complexity in the spec or the compiler; that's our problem.) It is easier to reason about "inference for local variables yes, fields and methods no" than adding various epicyclic considerations like "but, fields and methods are OK if they are private". Drawing the line where we did also means that the compatibility consequences of changing a field or method from private to nonprivate doesn't have accidental interactions with inference.

So the short answer is, doing it this way makes the language simpler, without making the feature dramatically less useful.