Why might one also use a blank constructor?

Question

I was reading some Java recently and came across something (an idiom?) new to me: in the program, classes with multiple constructors would also always include a blank constr

Answer 1

Having a default and empty (blank) constructor prevents you from having any final fields. This leads to a lot of mutability where there it is often not needed.

The builder pattern allows you to mix these two styles and allow more flexible initialization while still having immutability by hiding a many-arg constructor behind the factory.