How to create a list

Short answer

A modifiable list is an ArrayList. You can add and remove elements:

void main() {
    List<String> names = new ArrayList<>();
    names.add("Anna");
    names.add("Oleg");
 
    IO.println(names);
    IO.println(names.size());
}

Output:

[Anna, Oleg]
2

With elements right away

List.of() creates an unmodifiable list — handy for constants and read-only data:

void main() {
    List<String> days = List.of("Mon", "Tue", "Wed");
    IO.println(days.get(1));
}

Output:

Tue

Modifiable with initial elements

Pass List.of() to the ArrayList constructor — you get a modifiable copy:

void main() {
    List<Integer> numbers = new ArrayList<>(List.of(3, 1, 2));
    numbers.add(4);
    numbers.remove(Integer.valueOf(1));
 
    IO.println(numbers);
}

Output:

[3, 2, 4]

The type with var

With var the type is written only on the right:

void main() {
    var prices = new ArrayList<Double>();
    prices.add(9.99);
 
    IO.println(prices);
}

Output:

[9.99]

Common mistake

Modifying a list from List.of(). It is unmodifiable, and any attempt throws:

void main() {
    List<String> days = List.of("Mon", "Tue");
    days.add("Wed");
}

Output:

Exception in thread "main" java.lang.UnsupportedOperationException

Another detail: List.of() does not accept null elements.

Thanks for reading!

Short answer

With elements right away

Modifiable with initial elements

The type with var

Common mistake

Functions used in this recipe

Thanks for reading!