Maps: Map.of and Map.entry create immutable maps

Map.of is unmodifiable and rejects null keys and values. Its iteration order is deliberately unspecified, so wrap it in a TreeMap when you need to print it predictably.

Code

var config = Map.ofEntries(
        Map.entry("host", "localhost"),
        Map.entry("port", "8089"),
        Map.entry("ssl", "false"));

System.out.println(new TreeMap<>(config));

try {
    config.put("extra", "x");
} catch (UnsupportedOperationException e) {
    System.out.println("Map.of is unmodifiable");
}
try {
    Map.of("k", null);
} catch (NullPointerException e) {
    System.out.println("Map.of rejects null values");
}

Output

{host=localhost, port=8089, ssl=false}
Map.of is unmodifiable
Map.of rejects null values

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Published 2026-07-30