Immutable collections

Definition

List.of, Set.of and Map.of create collections that cannot be changed. List.copyOf makes an independent immutable snapshot, while Collections.unmodifiableList is only a read-only window onto a list that can still change.

Example code

List.copyOf is a snapshot, unmodifiableList is a live view

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;

class Demo {
    public static void main(String[] args) {
        List<String> source = new ArrayList<>(List.of("a", "b"));

        List<String> snapshot = List.copyOf(source);                 // independent copy
        List<String> view = Collections.unmodifiableList(source);    // live read-only view

        source.add("c");
        System.out.println("snapshot: " + snapshot);                   // unchanged
        System.out.println("view:     " + view);                       // shows the change

        Map<String, Integer> stock = Map.of("apple", 5, "pear", 2);
        try {
            stock.put("fig", 1);
        } catch (UnsupportedOperationException e) {
            System.out.println("Map.of is immutable");
        }
    }
}

// Output:
// snapshot: [a, b]
// view:     [a, b, c]
// Map.of is immutable