Question

I am looking for a relevant portion of the Java Language Specification (JLS) which describes the behaviour when invoking a variable arity (vararg) method.

Consider the method:

public static void printVarArgs(String... args) {
    System.out.println(Arrays.toString(args));
}

If I invoke the method like so:

printVarArgs();

The output will look like: [] because the omission of args at the call site has been converted into an empty array in the printVarArgs method.

I am looking for the point of the JLS which defines this behaviour. The closest I have found is 15.12.4.2 Evaluate Arguments, but it doesn’t give this example, and I’m not sure if this case is actually covered by the formal/mathematical description.

Which part of the JLS describes the automatic creation of an empty array when a vararg is omitted?

Answer 1

The text of that JLS section says:

If the method being invoked is a variable arity method (§8.4.1) m, it necessarily has n > 0 formal parameters. The final formal parameter of m necessarily has type T[] for some T, and m is necessarily being invoked with k >= 0 actual argument expressions.

If m is being invoked with kn actual argument expressions, or, if m is being invoked with k != n actual argument expressions and the type of the kth argument expression is not assignment compatible with T[], then the argument list (e1, ... , en-1, en, ...ek) is evaluated as if it were written as (e1, ..., en-1, new T[]{en, ..., ek}).

In the case you are talking about, there are k == n - 1 formal arguments, so en, ..., ek is an empty sequence, and that means the argument is evaluated as if it was (e1, ..., en-1, new T[]{}).

In other words, the behaviour is specified in the section you were looking at.