Are you saying that
you should never document default values??? I agree that if the attribute is internal with no way of changing from outside then you don't want to commit to an implementation. With the limited example in the OP there is no way to tell if this is the case or not.
However, it may be relevant information for a client of your API to know which constructor to use. Look at the available constructors of
HashMap. Are you saying that the designers got it wrong by documenting the default value for the no arg constructor:
Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
I suppose one alternative in this case may be to force the API client to always use one of the other constructors but that would defeat the purpose of having default values.