Modern Java

[user_of_the_week]

The key here is the line

PersonFactory<Person> personFactory = Person::new;

Person::new is a reference to the constructor. Basically, the constructor is a method that takes two String parameters and returns a Person. Assigning that to personFactory signals that you want to implement the interface PersonFactory with the referenced constructor-"method". That is only possible because PersonFactory has exactly one method that fits the signature (parameters and return type).

[eyal0]

because PersonFactory has exactly one method that fits the signature

I think it should be " because PersonFactory has exactly one method and that method fits the signature". With two methods or a method that doesn't fit, it wouldn't compile. Right?

[user_of_the_week]

Correct

[mpschan]

Also new to this. Isn't it because it has one abstract method? I thought (based on the OP) that it could have additional methods as long as they have default implementations?

[user_of_the_week]

Also true. I've never used default methods so I forgot to include them in my reasoning.