Java 9: Immutable collections with List.of / Map.of

Factory methods create compact, unmodifiable collections in one call.

Code

List<String> list = List.of("a", "b", "c");
Map<String, Integer> map = Map.of("x", 1, "y", 2);
System.out.println(list);
System.out.println(map.get("y"));
try { list.add("d"); } catch (UnsupportedOperationException e) {
    System.out.println("immutable: cannot add");
}

Output

[a, b, c]
2
immutable: cannot add

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Published 2026-07-26