How to convert a list to a map

Short answer

Collectors.toMap() takes two functions: how to get the key and how to get the value:

record User(int id, String name) {}
 
void main() {
    var users = List.of(new User(1, "Anna"), new User(2, "Oleg"));
 
    Map<Integer, String> names = users.stream().collect(Collectors.toMap(User::id, User::name));
    IO.println(names.get(2));
}

Output:

Oleg

The whole object as the value

Function.identity() returns the element itself — that is how you build an index by key:

record User(int id, String name) {}
 
void main() {
    var users = List.of(new User(1, "Anna"), new User(2, "Oleg"));
 
    Map<Integer, User> byId = users.stream().collect(Collectors.toMap(User::id, Function.identity()));
    IO.println(byId.get(1));
}

Output:

User[id=1, name=Anna]

Duplicate keys and order

The third argument decides which value to keep when keys collide, and the fourth sets the Map type:

void main() {
    var words = List.of("apple", "avocado", "banana", "blueberry", "cherry");
 
    var longestByLetter = words.stream().collect(Collectors.toMap(
            w -> w.charAt(0),
            w -> w,
            (a, b) -> a.length() >= b.length() ? a : b,
            LinkedHashMap::new));
    IO.println(longestByLetter);
}

Output:

{a=avocado, b=blueberry, c=cherry}

Common mistake

Not accounting for duplicate keys. Without a merge function, toMap() throws:

void main() {
    var words = List.of("apple", "avocado");
    words.stream().collect(Collectors.toMap(w -> w.charAt(0), w -> w));
}

Output:

Exception in thread "main" java.lang.IllegalStateException: Duplicate key a (attempted merging values apple and avocado)

Another trap: toMap() does not accept null values and throws NullPointerException.

Thanks for reading!

Short answer

The whole object as the value

Duplicate keys and order

Common mistake

Functions used in this recipe

Thanks for reading!