Batcher.java

public class Batcher<T> {
 
    private class Batch<T> {
 
        private final Collection<T> items = new ConcurrentLinkedQueue<>();
        private final AtomicInteger permits = new AtomicInteger();
 
        private Collection<T> getItems() {
            return items;
        }
 
        private AtomicInteger getPermits() {
            return permits;
        }
    }
 
    private volatile Batch<T> batch;
 
    public Batcher() {
        swap();
    }
 
    public Collection<T> swap() {
        Batch<T> currentBatch = batch;
        batch = new Batch<>();
        if (currentBatch != null) {
            AtomicInteger currentPermits = currentBatch.getPermits();
            for (; ; ) {
                if (currentPermits.get() == 0) {
                    return currentBatch.getItems();
                }
            }
        }
        return null;
    }
 
    public void add(T item) {
        Batch<T> currentBatch = batch;
        currentBatch.getPermits().incrementAndGet();
        currentBatch.getItems().add(item);
        currentBatch.getPermits().decrementAndGet();
    }
}