Java method dispatch with null argument

Question

Why does it (apparently) make a difference whether I pass null as an argument directly, or pass an Object that I assigned the value

Answer 1

Because the second commented out invocation with null is ambiguous to the compiler. The literal null could be a string or an object. Whereas the assigned value has a definite type. You need to cast the null, e.g. test.foo((String)null) to remove the ambiguity.