Throwing an exception that's exclusive to null arguments (whether NullPointerException or a custom type) makes automated null testing more reliable. This automated testing can be done with reflection and a set of default values, as in Guava's NullPointerTester. For example, NullPointerTester would attempt to call the following method...
Foo(String string, List list) {
checkArgument(string.length() > 0);
// missing null check for list!
this.string = string;
this.list = list;
}
...with two lists of arguments: "", null and null, ImmutableList.of(). It would test that each of these calls throws the expected NullPointerException. For this implementation, passing a null list does not produce NullPointerException. It does, however, happen to produce an IllegalArgumentException because NullPointerTester happens to use a default string of "". If NullPointerTester expects only NullPointerException for null values, it catches the bug. If it expects IllegalArgumentException, it misses it.
