Code snippet from The Coding Café - snippet-7.java

snippet-7.java

public static <T> Gatherer<T, ?, List<T>> 
        batchByCondition(Predicate<List<T>> batchComplete) {
    return Gatherer.of(
        ArrayList::new,  // 1️⃣ Current batch being built
        (batch, element, downstream) -> {
            batch.add(element);                        // 2️⃣ Add to current batch
            if (batchComplete.test(batch)) {           // 3️⃣ Check if batch is "full"
                downstream.push(new ArrayList<>(batch)); // 4️⃣ Send copy downstream
                batch.clear();                         // 5️⃣ Start fresh batch
            }
            return true;                               // 6️⃣ Keep processing
        },
        (batch, downstream) -> {  // 7️⃣ Finisher: handle remaining elements
            if (!batch.isEmpty()) {
                downstream.push(new ArrayList<>(batch));
            }
        }
    );
}

List<Integer> numbers = List.of(1, 4, 2, 8, 5, 7, 3, 9, 6);
List<List<Integer>> batches = numbers.stream()
    .gather(batchByCondition(
        batch -> batch.stream().mapToInt(i -> i).sum() <= 10  // Sum threshold
    ))
    .toList();

// Output visualization:
// [1, 4, 2] → Sum: 7    ✓ (7 ≤ 10, continue)
// [1, 4, 2, 8] → Sum: 15 ✗ (15 > 10, emit [1,4,2], start new with [8])
// [8] → Sum: 8          ✓ (8 ≤ 10, continue)
// [8, 5] → Sum: 13      ✗ (13 > 10, emit [8], start new with [5])
// ... and so on