- Objects.requireNonNull(T, Supplier) does not check for the Supplier
argument being null. Instead it relies on the fact, that the VM will
implicitly throw a NullPointerException when it calls .get on the
Supplier argument during the creation of the explicit
NullPointerException which it is supposed to throw.

- this behavior slightly differs from Objects.requireNonNull(T,
String) which simply creates a NullPointerException with a null
message in the case where the String argument is null.

- the difference is not evident in the OpenJDK, because the HotSpot VM
creates a NPE with a null message by default if we call a method on a
null object.

- however creating such a NPE with a null message when invoking a
method on a null object is not enforced by the standard, and other
implementations can do better :) For the following trivial program:

public class NonNull {
  public static void main(String[] args) {
    Supplier<String> ss = null;
    Object o = Objects.requireNonNull(null, ss);
  }
}

the current OpenJDK implementation returns:

Exception in thread "main" java.lang.NullPointerException
    at java.util.Objects.requireNonNull(Objects.java:290)
    at NonNull.main(NonNull.java:8)

but the SAP JVM will print:

Exception in thread "main" java.lang.NullPointerException: while
trying to invoke the method java.util.function.Supplier.get() of a
null object loaded from local variable 'messageSupplier'
    at java.util.Objects.requireNonNull(Objects.java:290)
    at NonNull.main(NonNull.java:8)

which is at least confusing for a call to Objects.requireNonNul() with
a null Supplier. We think the exception should be the same like the
one we get when invoking Objects.requireNonNul() with a null String.

- because of this difference (and because we like our extended
Exception messages :) the JCK test for Objects.requireNonNul(T,
Supplier) (i.e.
api/java_util/Objects/index.html#RequireNonNullMessageSupplier) was
removed from TCK 8 and is still commented out in TCK 9
(npe_checkingNullSupplier() in RequireNonNullMessageSupplier.java).

I really think that the simplest and most natural fix for this problem
is to simply check for a null Supplier argument and create the NPE
with an explicit null message in that case (as outlined in the
webrev). This:
 - makes the two requireNonNul() overloads for String and Supplier
behave the same (which I think was the initial intention).
 - doesn't require documentation changes
 - makes it possible to write a simple and conforming TCK test