How to transform every element of a list

Short answer

map() applies a function to every element and returns a stream of results:

void main() {
    var names = List.of("anna", "oleg");
    var upper = names.stream().map(String::toUpperCase).toList();
 
    IO.println(upper);
}

Output:

[ANNA, OLEG]

Extracting a field

A common case is getting a list of values of one field:

record User(String name, String email) {}
 
void main() {
    var users = List.of(new User("Anna", "anna@site.com"), new User("Oleg", "oleg@site.com"));
    IO.println(users.stream().map(User::email).toList());
}

Output:

[anna@site.com, oleg@site.com]

Numbers

mapToInt() gives a number stream with sum(), max() and average():

void main() {
    var words = List.of("java", "is", "fun");
    IO.println(words.stream().mapToInt(String::length).sum());
}

Output:

Changing in place

replaceAll() replaces every element of a modifiable list with the function's result:

void main() {
    var prices = new ArrayList<>(List.of(10, 20, 30));
    prices.replaceAll(p -> p * 2);
 
    IO.println(prices);
}

Output:

[20, 40, 60]

Common mistake

Using forEach() to fill another list. It works, but map() with toList() is shorter and needs no mutable list outside:

void main() {
    var names = List.of("anna", "oleg");
 
    var result = new ArrayList<String>();
    names.forEach(n -> result.add(n.toUpperCase()));
    IO.println(result);
 
    IO.println(names.stream().map(String::toUpperCase).toList());
}

Output:

[ANNA, OLEG]
[ANNA, OLEG]

Thanks for reading!

Short answer

Extracting a field

Numbers

Changing in place

Common mistake

Functions used in this recipe

Thanks for reading!