Java constructor fails to work with varargs

Question

I have the following enum which has a number of constructors:

public enum Route
{
   HOMEPAGE("", null, UserType.GUEST);

   Route(String baseName, String langFile, Entity entity) {}
   Route(String langFile, Entity entity)  {}
   Route(String langFile, UserType... availability) {}
   Route(String baseName, String langFile, UserType... availability) {}
}

In this case, I'm pretty clearly calling the 4th constructor when I define HOMEPAGE . But the problem is, I'm getting the error: Cannot resolve constructor Route(java.lang.String, null, com.foo.UserType) .

If I either remove the varags from the constructor, ie so it looks like:

   Route(String baseName, String langFile, UserType availability) {}

Or if I change the null when defining HOMEPAGE , ie:

   HOMEPAGE("", "", UserType.GUEST);

Then it works. But it doesn't make sense to me why that is. Why doesn't it detect that I'm calling the 4th constructor?

Answer 1

The problem is that null could be either a String or a UserType . So:

HOMEPAGE("", null, UserType.GUEST);

would match either the third or fourth constructor. Casting the null to a String would result in the fourth constructor being selected:

HOMEPAGE("", (String) null, UserType.GUEST);

Answer 2

It is correct that it does not know which constructor to pick, your call is:

Route(String, null, UserType)

This can surely not refer to the top two constructors.

It can however refer to Route(String, UserType) , as the null could be of type UserType .
It could also refer to Route(String, String, UserType) , as the null could be of type String .

There is your conflict.

Answer 3

Because null can either be a String or a UserType :

Route(java.lang.String, null, com.foo.UserType)

matches with both methodes :

Route(String langFile, UserType... availability) {}
Route(String baseName, String langFile, UserType... availability) {}