Question

I have been writing java for a while, and today I encountered the following declaration:

public static void main(String... args) {

}

Note the “dot dot dot” in the array declaration, rather than the usual bracket []. Clearly it works. In fact I wrote a small test and verified it works. So, I pulled the java grammar to see where this syntax of argument declaration is, but did not find anything.

So to the experts out there, how does this work? Is it part of the grammar? Also, while I can declare function like this, I can’t declare an array within a function’s body like this.

Anyway, do you know of any place that has this documented. It is curiosity, and perhaps not worth of any time invested in it, but I was stumped.

Answer 1

Check out the Java Language Specification, Third Edition, Chapter 8 (Classes). Buried in there is this nugget:

If the last formal parameter is a variable arity parameter of type T, it is considered to define a formal parameter of type T[]. The method is then a variable arity method. Otherwise, it is a fixed arity method. Invocations of a variable arity method may contain more actual argument expressions than formal parameters. All the actual argument expressions that do not correspond to the formal parameters preceding the variable arity parameter will be evaluated and the results stored into an array that will be passed to the method invocation (§15.12.4.2).

Basically, the last parameter of any method call can have T.... If it has that, it is converted to T[].

So basically, what you have is a fancy way of reproducing the more traditional

String[] args