Background: I'm currently working on a number of fixes for https://github.com/vorburger/xtendbeans (AKA AssertBeans), as Genius & netvirt developers are starting to pick up using this utility, respectively its adaptation to ODL known as AssertDataObjects from org.opendaylight.mdsal.binding.testutils org.opendaylight.mdsal:mdsal-binding-test-utils, because that utility does not yet work with "type union" YANG gen. code, such as e.g. the IpAddress type. (FYI the https://git.opendaylight.org/gerrit/#/c/50137/ has a non-regression type for the upcoming fixes in org.opendaylight.mdsal.binding.testutils / ch.vorburger.xtendbeans.)

While doing this work, I noticed that YANG gen. (stub) code for union type *Builder classes does not have a private constructor, as I would argue they all should have - even if users manually extend those *Builder classes - because users would be adding constructors with additional arguments, never implement the default constructor.

For example, the org.opendaylight.yang.gen.v1.urn.opendaylight.params.xml.ns.yang.yangtools.test.union.rev150121.UnionTestTypeBuilder found in org.opendaylight.mdsal:mdsal-binding-test-model:0.10.0-SNAPSHOT looks like this:

public class UnionTestTypeBuilder {
public static UnionTestType getDefaultInstance(java.lang.String defaultValue) {

but, IMHO, is missing this:

private UnionTestTypeBuilder() { }

I can see that e.g. IpAddressBuilder (from mdsal/model/ietf/ietf-inet-types) has such a private constructor, but that seems to have been manually added after that *Builder was originally generated. A number of other code. gen. and not manually edited union type *Builder classes do not have this private constructor - this bug suggests to extend the code generator templates to add this.

This would help make it clear to any developers that these kind of *Builder classes are never meant to be instantiated with their no-args default constructor.

BTW: Personally I find the naming of these *Builder classes a little unfortunate, because they look more like *Factory than *Builder - like they don't have any setter methods, don't implement org.opendaylight.yangtools.concepts.Builder and thus no build() - these really are a different concept, and having named them *Builder is more confusing than if they were simply named *Factory, or anything else other than *Builder - but it's probably way too late to change this - or is it?