Why does passing null to varargs give length 1?

Question

I've been using Java's variable arguments in my methods, and have been testing them by passing in null as a parameter:

public void method(Object A, Object...b) {
    if(A == null || b == null || b.length == 0) return;
...(something that invokes b)...

public void test() {
    method(new Object(), null)
...

At the beginning of the method, I checked for null as the parameter, and even for the length to be 0. But since the test kept failing because of a NullPointerException, I ran the debugger, and b was length 1! It even said when I clicked on the disclosure triangle for b,

all elements are null

I eventually resolved this by checking for length 1 and element 0 is null, but does anyone have an explanation for why Java does this? Thanks!

Answer 1

Because you're passing in a value of null.

If you want your varargs method to receive an empty array, either

1. pass that explicitly:

   method(new Object(), new Object[0])
   

2. don't pass anything at all:

   method(new Object())
   

Option 2 would be more idiomatic.

Answer 2

null is a value.

You provide a single value for the varargs argument, so the array will be new Object[] { null } which has a length of 1.

If you wanted to pass no values at all, you'd have to call method(new Object());.