Maps: computeIfAbsent builds a multimap without null checks

computeIfAbsent creates the missing value and returns it, replacing the get-check-put dance that new code should no longer contain.

Code

Map<String, List<String>> byFirstLetter = new TreeMap<>();
for (String name : List.of("Ava", "Ben", "Amit", "Cara", "Bala")) {
    byFirstLetter.computeIfAbsent(name.substring(0, 1), k -> new ArrayList<>()).add(name);
}
byFirstLetter.forEach((letter, names) -> System.out.println(letter + " -> " + names));

Output

A -> [Ava, Amit]
B -> [Ben, Bala]
C -> [Cara]

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Published 2026-07-30