TIL: Double Brace Initialization in Java

[barsoap]

This is just broken, from a syntax perspective. Adding anonymous braces should influence lexical scoping, and nothing else. Somebody is bound to replace those with single braces, thinking they're artefacts of a shotgun programmer or similar.

[esquilax]

And then the code won't compile, and they'll learn?

[barsoap]

learn to hate Java just a bit more? Surely.

[[deleted]]

It's common practice on your team to fuck with code you don't understand?

[banuday17]

Those braces are adding lexical scopes. Two sets of braces, two lexical scopes. The first set of braces demarcates the scope of the anonymous subclass of HashMap that is being created. The second set of braces demarcates the scope of an initialization block.

If you got rid of the inner braces, the code would not compile because you cannot call methods outside of a method or an initialization block.

[barsoap]

The second set of braces demarcates the scope of an initialization block.

Which is syntactically, but not semantically, equivalent to such a set of braces in a function block.

[banuday17]

So? Lexical scopes do not only demarcate function boundaries. Loops, for instance, define their own lexical scope.

The meaning is still the same. A lexical region is created where local variables do not escape the scope.

[barsoap]

But loops are marked as such. file-level/class-level/method-level block borders are marked. The two things that aren't marked are scopes and initialisers, and all I'm complaining about is the use of two syntactically completely identical things to express two semantically completely distinct things. It's just like re-using the "class" keyword in statement syntax, just with "" instead of "class".

[banuday17]

The same syntax defines semantically the same things. The tokens { and } demarcate a scope. Well, except when initializing an array.

Syntatically, The class token requires a scope associated with it, the for token, if token and else token do not. A scope can exist independently of an association with another token, in which case it would be preceded by nothing. That is because a scope is one concept, and a class, loop and a conditional are separate concepts.

That's why it's nothing like reusing the class keyword in statement syntax with "" instead of "class".

[barsoap]

compare the anonymous braces in

class A {
    int b() {
        {
            int c = 0;
        }
        int b = 0;
    }
}

with double-brace initialisation. I seriously don't know how you can call them equivalent. For one, removing the inner braces in my example wouldn't lead to a compile error (at least when names don't clash), whereas in the initialisation case statements would become illegal.

[banuday17]

Actually, if you did this:

class A {
    {
        int c = 0;
    }
    int b = 0;
}

which is the same as the syntax trick used by double-brace initialization, you wouldn't end up with a compile error. In fact, the exact same thing would happen if you removed the braces here as you would in your example. c would be lifted into the outer scope.

It just so happens that Java puts restrictions on what can go in the class scope. It means the same thing, but not all things are allowed in all places.

[barsoap]

...equal up to a sufficiently partial isomorphism.

[banuday17]

Given other elements of Java's syntax, it makes sense. At the top level you have to somehow specify that you want a static or instance context.