language design - Calling Java varargs method with single null argument?

Question

If I have a vararg Java method foo(Object ...arg) and I call foo(null, null), I have both arg[0] and arg[1] as nulls. But if I call foo(null), arg itself is null. Why is this happening?

How should I call foo such that foo.length == 1 && foo[0] == null is true?

Answer 1

The issue is that when you use the literal null, Java doesn't know what type it is supposed to be. It could be a null Object, or it could be a null Object array. For a single argument it assumes the latter.

You have two choices. Cast the null explicitly to Object or call the method using a strongly typed variable. See the example below:

public class Temp{
   public static void main(String[] args){
      foo("a", "b", "c");
      foo(null, null);
      foo((Object)null);
      Object bar = null;
      foo(bar);
   }

   private static void foo(Object...args) {
      System.out.println("foo called, args: " + asList(args));
   }
}

Output:

foo called, args: [a, b, c]
foo called, args: [null, null]
foo called, args: [null]
foo called, args: [null]