### Question:
AttributeHolderScope implements Scope, Provider<AHT> { public void enter(final AHT holder) { if (holder == null) { throw new IllegalArgumentException(); } this.holder.get().add(holder); }  void enter(final AHT holder); void exit(); @Override AHT get(); @Override final Provider<T> scope(
            final Key<T> key, final Provider<T> outer);  }### Answer:
@Test(expectedExceptions = IllegalArgumentException.class) public void cannotEnterWithNullHolder() { scope.enter(null); }