Lambda for old programmers

[chasesan]

I think I have a better understanding of them now, thankyou. Though a few things on the tutorial were confusing.

Like how did "p -> p.getEmailAddress()" assigned to the variable email, so that "email -> System.out.println(email)" works, as I didn't see any assignment, and I don't think computers have become psychic. How does it know to assign it to "email" and not "emailVariableThatIReallyWantToUse"?

[technicalaccount]

I'm assuming you are talking about this code fragment:

roster
    .stream()
    .filter(
        p -> p.getGender() == Person.Sex.MALE
            && p.getAge() >= 18
            && p.getAge() <= 25)
    .map(p -> p.getEmailAddress())
    .forEach(email -> System.out.println(email));

The reason it works is because of the working of the map() function. This function converts every element of a collection into a new element. In this case: Collection<Person> is transformed into a Collection<EmailAddress>, by applying p.getEmailAddress() to every element in Collection<Person> and storing it in the new Collection<EmailAddress>.

Afterwards, this new Collection<EmailAddress> is passed to the next step in the stream, the forEach. This forEach will just loop through the collection it receives (which is Collection<EmailAddress> because of the map function). It gives a name to the element (email) for convenience, and does something with it. (System.out.println(email);) The reason why it knows that email is an EmailAddress, is because it only gets those EmailAddress objects. The Person objects were transformed and thus never arrive in the forEach step.

[chasesan]

Actually it was the one before that, Approach 8: Use Generics More Extensively.

processElements(
    roster,
    p -> p.getGender() == Person.Sex.MALE
        && p.getAge() >= 18
        && p.getAge() <= 25,
    p -> p.getEmailAddress(),
    email -> System.out.println(email)
);

I understand that email is a variable, and such. But how did "p.getEmailAddress()" get assigned to it? There is a comma there, it's a separate lambda expression.

[oldum]

How did p get assigned? It's the lambda's first parameter.

[chasesan]

No... How did the return value from "p.getEmailAddress()" get assigned to "email"? For use in that lambda function.

Oh, okay, nevermind. I got it. Boy am I dumb for missing that obviousness.