Calling Java varargs method with single null argument?

java, language-design, null, variadic-functions

Solution

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]

Problem

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 `null`s. 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`?

Original source