copyOf and unmodifiable collectors

Definition

Java 10 added List.copyOf, Set.copyOf and Map.copyOf to make an immutable copy of an existing collection. It also added Collectors.toUnmodifiableList, toUnmodifiableSet and toUnmodifiableMap for streams.

Example code

Copying a list and collecting a stream into an immutable list

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

class Demo {
    public static void main(String[] args) {
        List<String> mutable = new ArrayList<>(List.of("a", "b"));
        List<String> copy = List.copyOf(mutable);          // immutable snapshot

        mutable.add("c");
        System.out.println(copy);                           // unchanged

        List<Integer> squares = List.of(1, 2, 3).stream()
                .map(n -> n * n)
                .collect(Collectors.toUnmodifiableList());
        System.out.println(squares);

        try {
            squares.add(16);
        } catch (UnsupportedOperationException e) {
            System.out.println("cannot change it");
        }
    }
}

// Output:
// [a, b]
// [1, 4, 9]
// cannot change it