Code snippet from The Coding Café - snippet-5.java

snippet-5.java

public static <T, K> Gatherer<T, ?, T> distinctByKey(Function<T, K> keyExtractor) {
    return Gatherer.of(
        HashSet::new,  // 1️⃣ State: Set to track seen keys
        (state, element, downstream) -> {
            K key = keyExtractor.apply(element);    // 2️⃣ Extract the key
            if (state.add(key)) {                   // 3️⃣ If key is new (add returns true)
                return downstream.push(element);     // 4️⃣ Pass element downstream
            }
            return true;                            // 5️⃣ Continue processing
        }
    );
}

List<String> words = List.of("hello", "world", "java", "gatherers", "stream", "api");
List<String> distinctByLength = words.stream()
    .gather(distinctByKey(String::length))      // Using length as the key
    .toList();

System.out.println("Distinct by length: " + distinctByLength);
// Output: [hello, java, gatherers, api]
//          ↑      ↑     ↑          ↑
//       len=5  len=4  len=9     len=3
// Note: "world"(5) and "stream"(6) are filtered out as duplicates