java - When to prefer a varargs list to an array?

Question

I'm implementing an API an have a method which you pass a list of paths where the program reads resources from

public void importFrom(String... paths) {

}

I'm using varargs to make calling the method as convenient as possible to the user, like so

obj.importFrom("/foo", "/foo/bar);

Is this an appropriate use of varargs? Or is passing in an array better?

Answer 1

In your case varargs is just fine. You don't really need to make an array of the paths that you will be importing because there's nothing you want to do with these paths other than to pass them along to your importFrom method.

The varargs functionality saves you from having to explicitly create an array solely for the purpose of passing a collection of values to a one-off method, which you do appear to have here.

BTW, you can still pass in an array if you want to

public class VarargsDemo {
    public static void f(String... args) {
        for (String s: args) {
            System.out.println(s);
        }
    }
    public static void main(String[] args) {
        String[] english = new String[]{"one", "two", "three"};
        f(english);
        f("uno", "dos", "tres");
    }
}

Because the behavior is the same, the difference comes down to a (probably minor) question of what you want the method signature to "say". When you declare a method to take an explicit array parameter, it's almost as if you want to stress that you want to operate on an array object, something that has been defined outside the method and has its own existence and importance outside the method, and one in which, perhaps, operations like indexing matter. When declaring the method with varargs, its as if you are saying "just give me a bunch of items".

Then again, this doesn't have to be true; the JVM doesn't know the difference, all it sees is an array at run time. Many programmers won't bother splitting hairs over the intent of the method signature. Varargs is all about making the calls convenient.

That said, the main limitation of varargs is that such a parameter must be the last one of the method. In your case this is not a problem, but in general it is something to consider.