@SuppressWarnings("unchecked")
	public Mapper<K, V> valueNotNull() {
		return addConstraint((MapConstraint<K, V>) ValueNotNullRestraint.INSTANCE);
	}
@ SuppressWarnings ( "unchecked" ) public Mapper < K , V > valueNotNull ( ) { return addConstraint ( ( MapConstraint < K , V > ) ValueNotNullRestraint . INSTANCE ) ; }
Add a constraint that verifies that the value is
null. If is null, a {@link NullPointerException} is thrown.

@return