Java: computeIfAbsent to build multimaps

computeIfAbsent inserts a default value only when a key is missing.

Code

Map<String, List<Integer>> groups = new HashMap<>();
int[][] pairs = {{1, 10}, {1, 11}, {2, 20}};
for (int[] p : pairs) {
    groups.computeIfAbsent(String.valueOf(p[0]), k -> new ArrayList<>()).add(p[1]);
}
System.out.println(groups);

Output

{1=[10, 11], 2=[20]}

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Published 2026-07-26