Streams: groupingBy with counting builds a frequency map

The classic word count in a single expression. TreeMap::new keeps keys sorted so the output is stable across runs.

Code

var words = List.of("java", "sql", "java", "kafka", "sql", "java");

Map<String, Long> freq = words.stream()
        .collect(Collectors.groupingBy(w -> w, TreeMap::new, Collectors.counting()));

freq.forEach((word, count) -> System.out.println(word + " -> " + count));

Output

java -> 3
kafka -> 1
sql -> 2

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Published 2026-07-30