Maps: unmodifiable maps from a stream with toUnmodifiableMap

Collectors.toUnmodifiableMap builds the map and seals it in one step, which is the safest thing to return from a public method.

Code

var raw = List.of("a:1", "b:2", "c:3");

Map<String, Integer> conf = raw.stream()
        .map(s -> s.split(":"))
        .collect(Collectors.toUnmodifiableMap(p -> p[0], p -> Integer.parseInt(p[1])));

System.out.println(new TreeMap<>(conf));
try {
    conf.put("d", 4);
} catch (UnsupportedOperationException e) {
    System.out.println("result is unmodifiable");
}

Output

{a=1, b=2, c=3}
result is unmodifiable

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Published 2026-07-30