Wich design pattern is this?

I read a lot about design patterns but i still got it difficult identifiying when i have to use them. Today I was reading oracle documentation about lambdas and saw a class "evolution" and said "Hey, clearly there's some decoupling here". I think there's a well known pattern here but don't know exactly wich is.

Another question that I also have about this is, if I'm not using SPRING where the folders structure about interfaces and implementations is very clear, wich would be -according goog practices- the project structure where I must create the interfaces.

The example started with this code:

public static void printPersonsOlderThan(List<Person> roster, int age) {
    for (Person p : roster) {
        if (p.getAge() >= age) {
            p.printPerson();
        }
    }
}

Then continued with this:

public static void printPersonsWithinAgeRange(
    List<Person> roster, int low, int high) {
    for (Person p : roster) {
        if (low <= p.getAge() && p.getAge() < high) {
            p.printPerson();
        }
    }
}

And ended with this:

public static void printPersons(
    List<Person> roster, CheckPerson tester) {
    for (Person p : roster) {
        if (tester.test(p)) {
            p.printPerson();
        }
    }
}

Created this interface:

interface CheckPerson {
    boolean test(Person p);
}

And this was the implementation:

class CheckPersonEligibleForSelectiveService implements CheckPerson {
    public boolean test(Person p) {
        return p.gender == Person.Sex.MALE &&
            p.getAge() >= 18 &&
            p.getAge() <= 25;
    }
}